[PATCH v11 0/3] Memory poison recovery in khugepaged collapsing

[PATCH v11 0/3] Memory poison recovery in khugepaged collapsing

[Jiaqi Yan]

Problem
=======
Memory DIMMs are subject to multi-bit flips, i.e. memory errors.
As memory size and density increase, the chances of and number of
memory errors increase. The increasing size and density of server
RAM in the data center and cloud have shown increased uncorrectable
memory errors. There are already mechanisms in the kernel to recover
from uncorrectable memory errors. This series of patches provides
the recovery mechanism for the particular kernel agent khugepaged
when it collapses memory pages.

Impact
======
The main reason we chose to make khugepaged collapsing tolerant of
memory failures was its high possibility of accessing poisoned memory
while performing functionally optional compaction actions.
Standard applications typically don't have strict requirements on
the size of its pages. So they are given 4K pages by the kernel.
The kernel is able to improve application performance by either

  1) giving applications 2M pages to begin with, or
  2) collapsing 4K pages into 2M pages when possible.

This collapsing operation is done by khugepaged, a kernel agent that
is constantly scanning memory. When collapsing 4K pages into a 2M page,
it must copy the data from the 4K pages into a physically contiguous
2M page. Therefore, as long as there exists one poisoned cache line in
collapsible 4K pages, khugepaged will eventually access it. The current
impact to users is a machine check exception triggered kernel panic.
However, khugepaged’s compaction operations are not functionally required
kernel actions. Therefore making khugepaged tolerant to poisoned memory
will greatly improve user experience.

This patch series is for cases where khugepaged is the first guy
that detects the memory errors on the poisoned pages. IOW, the pages
are not known to have memory errors when khugepaged collapsing gets to
them. In our observation, this happens frequently when the huge page
ratio of the system is relatively low, which is fairly common in
virtual machines running on cloud.

Solution
========
As stated before, it is less desirable to crash the system only because
khugepaged accesses poisoned pages while it is collapsing 4K pages.
The high level idea of this patch series is to skip the group of pages
(usually 512 4K-size pages) once khugepaged finds one of them is poisoned,
as these pages have become ineligible to be collapsed.

We are also careful to unwind operations khuagepaged has performed before
it detects memory failures. For example, before copying and collapsing
a group of anonymous pages into a huge page, the source pages will be
isolated and their page table is unlinked from their PMD. These operations
need to be undone in order to ensure these pages are not changed/lost from
the perspective of other threads (both user and kernel space). As for
file backed memory pages, there already exists a rollback case. This
patch just extends it so that khugepaged also correctly rolls back when
it fails to copy poisoned 4K pages.

Changelog
=========

v11 changes
- Incorporate feedbacks from Shi Yang <shy828301@gmail.com> and Hugh
  Dickins <hughd@google.com>
- Replace releasing pages for-loop with release_pte_pages in
  __collapse_huge_page_copy_failed.
- Rename pte_ptl to ptl in __collapse_huge_page_copy_succeeded.
- Fix a bug in __collapse_huge_page_copy_succeeded: ptep_clear should be
  used instead of pte_clear.
- Drop _address in __collapse_huge_page_copy_succeeded.
- Add smp_mb() before updating filemap_nr_thps_dec.
- Move `nr = thp_nr_pages()` closer to its references.
- Remove an unnecessary goto statement.
- This revision is rebased to mm-unstable at commit b4e1277ee31db
  ("xtensa: reword ARCH_FORCE_MAX_ORDER prompt and help text")

v10 changes
- Incorporate feedbacks from Kirill A. Shutemov
  <kirill.shutemov@linux.intel.com>
- Refactor the 2nd loop (after the loop for copying memory) into 2 helper
  functions, one for actions to take when copying succeeded, one for when
  copying failed due to #MC.
- Use copy_mc_user_highpage for anonymous memory.
- Introduce copy_mc_highpage and use it for file-backed memory.
- Rename the original PMD from `rollback` to `orig_pmd`.
- Some minor changes in comments, e.g. `normal page` to `raw page`.
- This revision is rebased to mm-unstable at commit df3ae4347aff9
  ("dma-buf: system_heap: avoid reclaim for order 4")

v9 changes
- Incorporate feedback from Andrew Morton <akpm@linux-foundation.org>
- Move copy_mc_highpage into khugepage.c as a static out-of-line
  function copy_mc_page.

v8 changes
- Incorporate feedbacks from Tony Luck <tony.luck@intel.com>
- Rename copy_highpage_mc to copy_mc_highpage.
- Update copy_mc_highpage with kmsan changes.
- Code style changes:
  1) copy_mc_highpage returns int as "copy" is an action and is consistent
     with copy_mc_user_highpage.
  2) __collapse_huge_page_copy returns scan_result(int) and is consistent
     with __collapse_huge_page_isolate/swapin.
  3) variables are declared in separate lines in collapse_file.

v7 changes
- Fix a bug "KASAN: stack-out-of-bounds Read in collapse_file". After
  copying all pages into the huge page, clear_highpage should use index
  instead of page->index.

v6 changes
- Address comments from Kirill Shutemov <kirill@shutemov.name>
- Rewrite __collapse_huge_page_copy to make rollback operations more
  clear to its reader.
- Add detailed test steps in each commit message.

v5 changes
- Rebase patches to mm-unstable at
  commit ffb39098bf87 ("Merge tag 'linux-kselftest-kunit-6.1-rc1' of
  git://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest").
- Resolves conflicts with:
  commit 2f55f070e5b8 ("mm/khugepaged: minor cleanup for collapse_file")
  commit 1baec203b77c ("mm/khugepaged: try to free transhuge swapcache
  when possible")

v4 changes
- Incorporate feedbacks from Yang Shi <shy828301@gmail.com>
- Remove tracepoint for __collapse_huge_page_copy, just keep SCAN_COPY_MC
  and let trace_mm_collapse_huge_page it
- Remove unnecessary comments

v3 changes
- Incorporate feedbacks from Yang Shi <shy828301@gmail.com>
- Add tracepoint for __collapse_huge_page_copy
- Restore PMD in collapse_huge_page
- Correct comment about mmap_read_lock

v2 changes
- Incorporate feedbacks from Yang Shi <shy828301@gmail.com>
- Only keep copy_highpage_mc
- Adding new scan_result SCAN_COPY_MC
- Defer NR_FILE_THPS update until copying succeeded

Jiaqi Yan (3):
  mm/khugepaged: recover from poisoned anonymous memory
  mm/hwpoison: introduce copy_mc_highpage
  mm/khugepaged: recover from poisoned file-backed memory

 include/linux/highmem.h            |  54 ++++++--
 include/trace/events/huge_memory.h |   3 +-
 mm/khugepaged.c                    | 200 ++++++++++++++++++++++-------
 3 files changed, 198 insertions(+), 59 deletions(-)

-- 
2.40.0.348.gf938b09366-goog

[PATCH v11 1/3] mm/khugepaged: recover from poisoned anonymous memory

[Jiaqi Yan]

Make __collapse_huge_page_copy return whether copying anonymous pages
succeeded, and make collapse_huge_page handle the return status.

Break existing PTE scan loop into two for-loops. The first loop copies
source pages into target huge page, and can fail gracefully when running
into memory errors in source pages. If copying all pages succeeds, the
second loop releases and clears up these normal pages. Otherwise, the
second loop rolls back the page table and page states by:
- re-establishing the original PTEs-to-PMD connection.
- releasing source pages back to their LRU list.

Tested manually:
0. Enable khugepaged on system under test.
1. Start a two-thread application. Each thread allocates a chunk of
   non-huge anonymous memory buffer.
2. Pick 4 random buffer locations (2 in each thread) and inject
   uncorrectable memory errors at corresponding physical addresses.
3. Signal both threads to make their memory buffer collapsible, i.e.
   calling madvise(MADV_HUGEPAGE).
4. Wait and check kernel log: khugepaged is able to recover from poisoned
   pages and skips collapsing them.
5. Signal both threads to inspect their buffer contents and make sure no
   data corruption.

Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
---
 include/trace/events/huge_memory.h |   3 +-
 mm/khugepaged.c                    | 114 +++++++++++++++++++++++++----
 2 files changed, 103 insertions(+), 14 deletions(-)

diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index 3e6fb05852f9a..46cce509957ba 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -36,7 +36,8 @@
 	EM( SCAN_ALLOC_HUGE_PAGE_FAIL,	"alloc_huge_page_failed")	\
 	EM( SCAN_CGROUP_CHARGE_FAIL,	"ccgroup_charge_failed")	\
 	EM( SCAN_TRUNCATED,		"truncated")			\
-	EMe(SCAN_PAGE_HAS_PRIVATE,	"page_has_private")		\
+	EM( SCAN_PAGE_HAS_PRIVATE,	"page_has_private")		\
+	EMe(SCAN_COPY_MC,		"copy_poisoned_page")		\
 
 #undef EM
 #undef EMe
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index bee7fd7db380a..bef68286345c8 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -55,6 +55,7 @@ enum scan_result {
 	SCAN_CGROUP_CHARGE_FAIL,
 	SCAN_TRUNCATED,
 	SCAN_PAGE_HAS_PRIVATE,
+	SCAN_COPY_MC,
 };
 
 #define CREATE_TRACE_POINTS
@@ -681,20 +682,22 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	return result;
 }
 
-static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
-				      struct vm_area_struct *vma,
-				      unsigned long address,
-				      spinlock_t *ptl,
-				      struct list_head *compound_pagelist)
+static void __collapse_huge_page_copy_succeeded(pte_t *pte,
+						pmd_t *pmd,
+						struct vm_area_struct *vma,
+						unsigned long address,
+						spinlock_t *ptl,
+						struct list_head *compound_pagelist)
 {
-	struct page *src_page, *tmp;
+	struct page *src_page;
+	struct page *tmp;
 	pte_t *_pte;
-	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
-				_pte++, page++, address += PAGE_SIZE) {
-		pte_t pteval = *_pte;
+	pte_t pteval;
 
+	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
+	     _pte++, address += PAGE_SIZE) {
+		pteval = *_pte;
 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
-			clear_user_highpage(page, address);
 			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
 			if (is_zero_pfn(pte_pfn(pteval))) {
 				/*
@@ -706,7 +709,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
 			}
 		} else {
 			src_page = pte_page(pteval);
-			copy_user_highpage(page, src_page, address, vma);
 			if (!PageCompound(src_page))
 				release_pte_page(src_page);
 			/*
@@ -733,6 +735,88 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
 	}
 }
 
+static void __collapse_huge_page_copy_failed(pte_t *pte,
+					     pmd_t *pmd,
+					     pmd_t orig_pmd,
+					     struct vm_area_struct *vma,
+					     unsigned long address,
+					     struct list_head *compound_pagelist)
+{
+	spinlock_t *pmd_ptl;
+
+	/*
+	 * Re-establish the PMD to point to the original page table
+	 * entry. Restoring PMD needs to be done prior to releasing
+	 * pages. Since pages are still isolated and locked here,
+	 * acquiring anon_vma_lock_write is unnecessary.
+	 */
+	pmd_ptl = pmd_lock(vma->vm_mm, pmd);
+	pmd_populate(vma->vm_mm, pmd, pmd_pgtable(orig_pmd));
+	spin_unlock(pmd_ptl);
+	/*
+	 * Release both raw and compound pages isolated
+	 * in __collapse_huge_page_isolate.
+	 */
+	release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
+}
+
+/*
+ * __collapse_huge_page_copy - attempts to copy memory contents from raw
+ * pages to a hugepage. Cleans up the raw pages if copying succeeds;
+ * otherwise restores the original page table and releases isolated raw pages.
+ * Returns SCAN_SUCCEED if copying succeeds, otherwise returns SCAN_COPY_MC.
+ *
+ * @pte: starting of the PTEs to copy from
+ * @page: the new hugepage to copy contents to
+ * @pmd: pointer to the new hugepage's PMD
+ * @orig_pmd: the original raw pages' PMD
+ * @vma: the original raw pages' virtual memory area
+ * @address: starting address to copy
+ * @pte_ptl: lock on raw pages' PTEs
+ * @compound_pagelist: list that stores compound pages
+ */
+static int __collapse_huge_page_copy(pte_t *pte,
+				     struct page *page,
+				     pmd_t *pmd,
+				     pmd_t orig_pmd,
+				     struct vm_area_struct *vma,
+				     unsigned long address,
+				     spinlock_t *pte_ptl,
+				     struct list_head *compound_pagelist)
+{
+	struct page *src_page;
+	pte_t *_pte;
+	pte_t pteval;
+	unsigned long _address;
+	int result = SCAN_SUCCEED;
+
+	/*
+	 * Copying pages' contents is subject to memory poison at any iteration.
+	 */
+	for (_pte = pte, _address = address; _pte < pte + HPAGE_PMD_NR;
+	     _pte++, page++, _address += PAGE_SIZE) {
+		pteval = *_pte;
+		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
+			clear_user_highpage(page, _address);
+			continue;
+		}
+		src_page = pte_page(pteval);
+		if (copy_mc_user_highpage(page, src_page, _address, vma) > 0) {
+			result = SCAN_COPY_MC;
+			break;
+		}
+	}
+
+	if (likely(result == SCAN_SUCCEED))
+		__collapse_huge_page_copy_succeeded(pte, pmd, vma, address,
+						    pte_ptl, compound_pagelist);
+	else
+		__collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
+						 address, compound_pagelist);
+
+	return result;
+}
+
 static void khugepaged_alloc_sleep(void)
 {
 	DEFINE_WAIT(wait);
@@ -1106,9 +1190,13 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	 */
 	anon_vma_unlock_write(vma->anon_vma);
 
-	__collapse_huge_page_copy(pte, hpage, vma, address, pte_ptl,
-				  &compound_pagelist);
+	result = __collapse_huge_page_copy(pte, hpage, pmd, _pmd,
+					   vma, address, pte_ptl,
+					   &compound_pagelist);
 	pte_unmap(pte);
+	if (unlikely(result != SCAN_SUCCEED))
+		goto out_up_write;
+
 	/*
 	 * spin_lock() below is not the equivalent of smp_wmb(), but
 	 * the smp_wmb() inside __SetPageUptodate() can be reused to
-- 
2.40.0.348.gf938b09366-goog

Re: [PATCH v11 1/3] mm/khugepaged: recover from poisoned anonymous memory

[Yang Shi]

On Mon, Mar 27, 2023 at 2:15 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
>
> Make __collapse_huge_page_copy return whether copying anonymous pages
> succeeded, and make collapse_huge_page handle the return status.
>
> Break existing PTE scan loop into two for-loops. The first loop copies
> source pages into target huge page, and can fail gracefully when running
> into memory errors in source pages. If copying all pages succeeds, the
> second loop releases and clears up these normal pages. Otherwise, the
> second loop rolls back the page table and page states by:
> - re-establishing the original PTEs-to-PMD connection.
> - releasing source pages back to their LRU list.
>
> Tested manually:
> 0. Enable khugepaged on system under test.
> 1. Start a two-thread application. Each thread allocates a chunk of
>    non-huge anonymous memory buffer.
> 2. Pick 4 random buffer locations (2 in each thread) and inject
>    uncorrectable memory errors at corresponding physical addresses.
> 3. Signal both threads to make their memory buffer collapsible, i.e.
>    calling madvise(MADV_HUGEPAGE).
> 4. Wait and check kernel log: khugepaged is able to recover from poisoned
>    pages and skips collapsing them.
> 5. Signal both threads to inspect their buffer contents and make sure no
>    data corruption.
>
> Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>

Reviewed-by: Yang Shi <shy828301@gmail.com>

Just a nit below:

> ---
>  include/trace/events/huge_memory.h |   3 +-
>  mm/khugepaged.c                    | 114 +++++++++++++++++++++++++----
>  2 files changed, 103 insertions(+), 14 deletions(-)
>
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index 3e6fb05852f9a..46cce509957ba 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -36,7 +36,8 @@
>         EM( SCAN_ALLOC_HUGE_PAGE_FAIL,  "alloc_huge_page_failed")       \
>         EM( SCAN_CGROUP_CHARGE_FAIL,    "ccgroup_charge_failed")        \
>         EM( SCAN_TRUNCATED,             "truncated")                    \
> -       EMe(SCAN_PAGE_HAS_PRIVATE,      "page_has_private")             \
> +       EM( SCAN_PAGE_HAS_PRIVATE,      "page_has_private")             \
> +       EMe(SCAN_COPY_MC,               "copy_poisoned_page")           \
>
>  #undef EM
>  #undef EMe
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index bee7fd7db380a..bef68286345c8 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -55,6 +55,7 @@ enum scan_result {
>         SCAN_CGROUP_CHARGE_FAIL,
>         SCAN_TRUNCATED,
>         SCAN_PAGE_HAS_PRIVATE,
> +       SCAN_COPY_MC,
>  };
>
>  #define CREATE_TRACE_POINTS
> @@ -681,20 +682,22 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>         return result;
>  }
>
> -static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> -                                     struct vm_area_struct *vma,
> -                                     unsigned long address,
> -                                     spinlock_t *ptl,
> -                                     struct list_head *compound_pagelist)
> +static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> +                                               pmd_t *pmd,
> +                                               struct vm_area_struct *vma,
> +                                               unsigned long address,
> +                                               spinlock_t *ptl,
> +                                               struct list_head *compound_pagelist)
>  {
> -       struct page *src_page, *tmp;
> +       struct page *src_page;
> +       struct page *tmp;
>         pte_t *_pte;
> -       for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> -                               _pte++, page++, address += PAGE_SIZE) {
> -               pte_t pteval = *_pte;
> +       pte_t pteval;
>
> +       for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> +            _pte++, address += PAGE_SIZE) {
> +               pteval = *_pte;
>                 if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> -                       clear_user_highpage(page, address);
>                         add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
>                         if (is_zero_pfn(pte_pfn(pteval))) {
>                                 /*
> @@ -706,7 +709,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
>                         }
>                 } else {
>                         src_page = pte_page(pteval);
> -                       copy_user_highpage(page, src_page, address, vma);
>                         if (!PageCompound(src_page))
>                                 release_pte_page(src_page);
>                         /*
> @@ -733,6 +735,88 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
>         }
>  }
>
> +static void __collapse_huge_page_copy_failed(pte_t *pte,
> +                                            pmd_t *pmd,
> +                                            pmd_t orig_pmd,
> +                                            struct vm_area_struct *vma,
> +                                            unsigned long address,

It looks like "address" is not used at all. It could be removed.

> +                                            struct list_head *compound_pagelist)
> +{
> +       spinlock_t *pmd_ptl;
> +
> +       /*
> +        * Re-establish the PMD to point to the original page table
> +        * entry. Restoring PMD needs to be done prior to releasing
> +        * pages. Since pages are still isolated and locked here,
> +        * acquiring anon_vma_lock_write is unnecessary.
> +        */
> +       pmd_ptl = pmd_lock(vma->vm_mm, pmd);
> +       pmd_populate(vma->vm_mm, pmd, pmd_pgtable(orig_pmd));
> +       spin_unlock(pmd_ptl);
> +       /*
> +        * Release both raw and compound pages isolated
> +        * in __collapse_huge_page_isolate.
> +        */
> +       release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
> +}
> +
> +/*
> + * __collapse_huge_page_copy - attempts to copy memory contents from raw
> + * pages to a hugepage. Cleans up the raw pages if copying succeeds;
> + * otherwise restores the original page table and releases isolated raw pages.
> + * Returns SCAN_SUCCEED if copying succeeds, otherwise returns SCAN_COPY_MC.
> + *
> + * @pte: starting of the PTEs to copy from
> + * @page: the new hugepage to copy contents to
> + * @pmd: pointer to the new hugepage's PMD
> + * @orig_pmd: the original raw pages' PMD
> + * @vma: the original raw pages' virtual memory area
> + * @address: starting address to copy
> + * @pte_ptl: lock on raw pages' PTEs
> + * @compound_pagelist: list that stores compound pages
> + */
> +static int __collapse_huge_page_copy(pte_t *pte,
> +                                    struct page *page,
> +                                    pmd_t *pmd,
> +                                    pmd_t orig_pmd,
> +                                    struct vm_area_struct *vma,
> +                                    unsigned long address,
> +                                    spinlock_t *pte_ptl,
> +                                    struct list_head *compound_pagelist)
> +{
> +       struct page *src_page;
> +       pte_t *_pte;
> +       pte_t pteval;
> +       unsigned long _address;
> +       int result = SCAN_SUCCEED;
> +
> +       /*
> +        * Copying pages' contents is subject to memory poison at any iteration.
> +        */
> +       for (_pte = pte, _address = address; _pte < pte + HPAGE_PMD_NR;
> +            _pte++, page++, _address += PAGE_SIZE) {
> +               pteval = *_pte;
> +               if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> +                       clear_user_highpage(page, _address);
> +                       continue;
> +               }
> +               src_page = pte_page(pteval);
> +               if (copy_mc_user_highpage(page, src_page, _address, vma) > 0) {
> +                       result = SCAN_COPY_MC;
> +                       break;
> +               }
> +       }
> +
> +       if (likely(result == SCAN_SUCCEED))
> +               __collapse_huge_page_copy_succeeded(pte, pmd, vma, address,
> +                                                   pte_ptl, compound_pagelist);
> +       else
> +               __collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
> +                                                address, compound_pagelist);
> +
> +       return result;
> +}
> +
>  static void khugepaged_alloc_sleep(void)
>  {
>         DEFINE_WAIT(wait);
> @@ -1106,9 +1190,13 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>          */
>         anon_vma_unlock_write(vma->anon_vma);
>
> -       __collapse_huge_page_copy(pte, hpage, vma, address, pte_ptl,
> -                                 &compound_pagelist);
> +       result = __collapse_huge_page_copy(pte, hpage, pmd, _pmd,
> +                                          vma, address, pte_ptl,
> +                                          &compound_pagelist);
>         pte_unmap(pte);
> +       if (unlikely(result != SCAN_SUCCEED))
> +               goto out_up_write;
> +
>         /*
>          * spin_lock() below is not the equivalent of smp_wmb(), but
>          * the smp_wmb() inside __SetPageUptodate() can be reused to
> --
> 2.40.0.348.gf938b09366-goog
>

Re: [PATCH v11 1/3] mm/khugepaged: recover from poisoned anonymous memory

[Jiaqi Yan]

On Tue, Mar 28, 2023 at 8:59 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Mon, Mar 27, 2023 at 2:15 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
> >
> > Make __collapse_huge_page_copy return whether copying anonymous pages
> > succeeded, and make collapse_huge_page handle the return status.
> >
> > Break existing PTE scan loop into two for-loops. The first loop copies
> > source pages into target huge page, and can fail gracefully when running
> > into memory errors in source pages. If copying all pages succeeds, the
> > second loop releases and clears up these normal pages. Otherwise, the
> > second loop rolls back the page table and page states by:
> > - re-establishing the original PTEs-to-PMD connection.
> > - releasing source pages back to their LRU list.
> >
> > Tested manually:
> > 0. Enable khugepaged on system under test.
> > 1. Start a two-thread application. Each thread allocates a chunk of
> >    non-huge anonymous memory buffer.
> > 2. Pick 4 random buffer locations (2 in each thread) and inject
> >    uncorrectable memory errors at corresponding physical addresses.
> > 3. Signal both threads to make their memory buffer collapsible, i.e.
> >    calling madvise(MADV_HUGEPAGE).
> > 4. Wait and check kernel log: khugepaged is able to recover from poisoned
> >    pages and skips collapsing them.
> > 5. Signal both threads to inspect their buffer contents and make sure no
> >    data corruption.
> >
> > Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
>
> Reviewed-by: Yang Shi <shy828301@gmail.com>
>
> Just a nit below:
>
> > ---
> >  include/trace/events/huge_memory.h |   3 +-
> >  mm/khugepaged.c                    | 114 +++++++++++++++++++++++++----
> >  2 files changed, 103 insertions(+), 14 deletions(-)
> >
> > diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> > index 3e6fb05852f9a..46cce509957ba 100644
> > --- a/include/trace/events/huge_memory.h
> > +++ b/include/trace/events/huge_memory.h
> > @@ -36,7 +36,8 @@
> >         EM( SCAN_ALLOC_HUGE_PAGE_FAIL,  "alloc_huge_page_failed")       \
> >         EM( SCAN_CGROUP_CHARGE_FAIL,    "ccgroup_charge_failed")        \
> >         EM( SCAN_TRUNCATED,             "truncated")                    \
> > -       EMe(SCAN_PAGE_HAS_PRIVATE,      "page_has_private")             \
> > +       EM( SCAN_PAGE_HAS_PRIVATE,      "page_has_private")             \
> > +       EMe(SCAN_COPY_MC,               "copy_poisoned_page")           \
> >
> >  #undef EM
> >  #undef EMe
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index bee7fd7db380a..bef68286345c8 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -55,6 +55,7 @@ enum scan_result {
> >         SCAN_CGROUP_CHARGE_FAIL,
> >         SCAN_TRUNCATED,
> >         SCAN_PAGE_HAS_PRIVATE,
> > +       SCAN_COPY_MC,
> >  };
> >
> >  #define CREATE_TRACE_POINTS
> > @@ -681,20 +682,22 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >         return result;
> >  }
> >
> > -static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> > -                                     struct vm_area_struct *vma,
> > -                                     unsigned long address,
> > -                                     spinlock_t *ptl,
> > -                                     struct list_head *compound_pagelist)
> > +static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> > +                                               pmd_t *pmd,
> > +                                               struct vm_area_struct *vma,
> > +                                               unsigned long address,
> > +                                               spinlock_t *ptl,
> > +                                               struct list_head *compound_pagelist)
> >  {
> > -       struct page *src_page, *tmp;
> > +       struct page *src_page;
> > +       struct page *tmp;
> >         pte_t *_pte;
> > -       for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> > -                               _pte++, page++, address += PAGE_SIZE) {
> > -               pte_t pteval = *_pte;
> > +       pte_t pteval;
> >
> > +       for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> > +            _pte++, address += PAGE_SIZE) {
> > +               pteval = *_pte;
> >                 if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > -                       clear_user_highpage(page, address);
> >                         add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> >                         if (is_zero_pfn(pte_pfn(pteval))) {
> >                                 /*
> > @@ -706,7 +709,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> >                         }
> >                 } else {
> >                         src_page = pte_page(pteval);
> > -                       copy_user_highpage(page, src_page, address, vma);
> >                         if (!PageCompound(src_page))
> >                                 release_pte_page(src_page);
> >                         /*
> > @@ -733,6 +735,88 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> >         }
> >  }
> >
> > +static void __collapse_huge_page_copy_failed(pte_t *pte,
> > +                                            pmd_t *pmd,
> > +                                            pmd_t orig_pmd,
> > +                                            struct vm_area_struct *vma,
> > +                                            unsigned long address,
>
> It looks like "address" is not used at all. It could be removed.

Thanks for catching this!

pmd in __collapse_huge_page_copy_succeeded can also be dropped.

>
> > +                                            struct list_head *compound_pagelist)
> > +{
> > +       spinlock_t *pmd_ptl;
> > +
> > +       /*
> > +        * Re-establish the PMD to point to the original page table
> > +        * entry. Restoring PMD needs to be done prior to releasing
> > +        * pages. Since pages are still isolated and locked here,
> > +        * acquiring anon_vma_lock_write is unnecessary.
> > +        */
> > +       pmd_ptl = pmd_lock(vma->vm_mm, pmd);
> > +       pmd_populate(vma->vm_mm, pmd, pmd_pgtable(orig_pmd));
> > +       spin_unlock(pmd_ptl);
> > +       /*
> > +        * Release both raw and compound pages isolated
> > +        * in __collapse_huge_page_isolate.
> > +        */
> > +       release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
> > +}
> > +
> > +/*
> > + * __collapse_huge_page_copy - attempts to copy memory contents from raw
> > + * pages to a hugepage. Cleans up the raw pages if copying succeeds;
> > + * otherwise restores the original page table and releases isolated raw pages.
> > + * Returns SCAN_SUCCEED if copying succeeds, otherwise returns SCAN_COPY_MC.
> > + *
> > + * @pte: starting of the PTEs to copy from
> > + * @page: the new hugepage to copy contents to
> > + * @pmd: pointer to the new hugepage's PMD
> > + * @orig_pmd: the original raw pages' PMD
> > + * @vma: the original raw pages' virtual memory area
> > + * @address: starting address to copy
> > + * @pte_ptl: lock on raw pages' PTEs
> > + * @compound_pagelist: list that stores compound pages
> > + */
> > +static int __collapse_huge_page_copy(pte_t *pte,
> > +                                    struct page *page,
> > +                                    pmd_t *pmd,
> > +                                    pmd_t orig_pmd,
> > +                                    struct vm_area_struct *vma,
> > +                                    unsigned long address,
> > +                                    spinlock_t *pte_ptl,
> > +                                    struct list_head *compound_pagelist)
> > +{
> > +       struct page *src_page;
> > +       pte_t *_pte;
> > +       pte_t pteval;
> > +       unsigned long _address;
> > +       int result = SCAN_SUCCEED;
> > +
> > +       /*
> > +        * Copying pages' contents is subject to memory poison at any iteration.
> > +        */
> > +       for (_pte = pte, _address = address; _pte < pte + HPAGE_PMD_NR;
> > +            _pte++, page++, _address += PAGE_SIZE) {
> > +               pteval = *_pte;
> > +               if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > +                       clear_user_highpage(page, _address);
> > +                       continue;
> > +               }
> > +               src_page = pte_page(pteval);
> > +               if (copy_mc_user_highpage(page, src_page, _address, vma) > 0) {
> > +                       result = SCAN_COPY_MC;
> > +                       break;
> > +               }
> > +       }
> > +
> > +       if (likely(result == SCAN_SUCCEED))
> > +               __collapse_huge_page_copy_succeeded(pte, pmd, vma, address,
> > +                                                   pte_ptl, compound_pagelist);
> > +       else
> > +               __collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
> > +                                                address, compound_pagelist);
> > +
> > +       return result;
> > +}
> > +
> >  static void khugepaged_alloc_sleep(void)
> >  {
> >         DEFINE_WAIT(wait);
> > @@ -1106,9 +1190,13 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >          */
> >         anon_vma_unlock_write(vma->anon_vma);
> >
> > -       __collapse_huge_page_copy(pte, hpage, vma, address, pte_ptl,
> > -                                 &compound_pagelist);
> > +       result = __collapse_huge_page_copy(pte, hpage, pmd, _pmd,
> > +                                          vma, address, pte_ptl,
> > +                                          &compound_pagelist);
> >         pte_unmap(pte);
> > +       if (unlikely(result != SCAN_SUCCEED))
> > +               goto out_up_write;
> > +
> >         /*
> >          * spin_lock() below is not the equivalent of smp_wmb(), but
> >          * the smp_wmb() inside __SetPageUptodate() can be reused to
> > --
> > 2.40.0.348.gf938b09366-goog
> >

[PATCH v11 2/3] mm/hwpoison: introduce copy_mc_highpage

[Jiaqi Yan]

Similar to how copy_mc_user_highpage is implemented for
copy_user_highpage on #MC supported architecture, introduce
the #MC handled version of copy_highpage.

This helper has immediate usage when khugepaged wants to copy
file-backed memory pages and tolerate #MC.

Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
---
 include/linux/highmem.h | 54 +++++++++++++++++++++++++++++++----------
 1 file changed, 41 insertions(+), 13 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 9c7cdaa3de8cd..4de1dbcd3ef64 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -315,7 +315,29 @@ static inline void copy_user_highpage(struct page *to, struct page *from,
 
 #endif
 
+#ifndef __HAVE_ARCH_COPY_HIGHPAGE
+
+static inline void copy_highpage(struct page *to, struct page *from)
+{
+	char *vfrom, *vto;
+
+	vfrom = kmap_local_page(from);
+	vto = kmap_local_page(to);
+	copy_page(vto, vfrom);
+	kmsan_copy_page_meta(to, from);
+	kunmap_local(vto);
+	kunmap_local(vfrom);
+}
+
+#endif
+
 #ifdef copy_mc_to_kernel
+/*
+ * If architecture supports machine check exception handling, define the
+ * #MC versions of copy_user_highpage and copy_highpage. They copy a memory
+ * page with #MC in source page (@from) handled, and return the number
+ * of bytes not copied if there was a #MC, otherwise 0 for success.
+ */
 static inline int copy_mc_user_highpage(struct page *to, struct page *from,
 					unsigned long vaddr, struct vm_area_struct *vma)
 {
@@ -332,29 +354,35 @@ static inline int copy_mc_user_highpage(struct page *to, struct page *from,
 
 	return ret;
 }
-#else
-static inline int copy_mc_user_highpage(struct page *to, struct page *from,
-					unsigned long vaddr, struct vm_area_struct *vma)
-{
-	copy_user_highpage(to, from, vaddr, vma);
-	return 0;
-}
-#endif
 
-#ifndef __HAVE_ARCH_COPY_HIGHPAGE
-
-static inline void copy_highpage(struct page *to, struct page *from)
+static inline int copy_mc_highpage(struct page *to, struct page *from)
 {
+	unsigned long ret;
 	char *vfrom, *vto;
 
 	vfrom = kmap_local_page(from);
 	vto = kmap_local_page(to);
-	copy_page(vto, vfrom);
-	kmsan_copy_page_meta(to, from);
+	ret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);
+	if (!ret)
+		kmsan_copy_page_meta(to, from);
 	kunmap_local(vto);
 	kunmap_local(vfrom);
+
+	return ret;
+}
+#else
+static inline int copy_mc_user_highpage(struct page *to, struct page *from,
+					unsigned long vaddr, struct vm_area_struct *vma)
+{
+	copy_user_highpage(to, from, vaddr, vma);
+	return 0;
 }
 
+static inline int copy_mc_highpage(struct page *to, struct page *from)
+{
+	copy_highpage(to, from);
+	return 0;
+}
 #endif
 
 static inline void memcpy_page(struct page *dst_page, size_t dst_off,
-- 
2.40.0.348.gf938b09366-goog

Re: [PATCH v11 2/3] mm/hwpoison: introduce copy_mc_highpage

[Yang Shi]

On Mon, Mar 27, 2023 at 2:15 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
>
> Similar to how copy_mc_user_highpage is implemented for
> copy_user_highpage on #MC supported architecture, introduce
> the #MC handled version of copy_highpage.
>
> This helper has immediate usage when khugepaged wants to copy
> file-backed memory pages and tolerate #MC.
>
> Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>

Reviewed-by: Yang Shi <shy828301@gmail.com>

BTW, you could preserve the reviewed/acked tags as long as there is no
significant change between two versions.

> ---
>  include/linux/highmem.h | 54 +++++++++++++++++++++++++++++++----------
>  1 file changed, 41 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 9c7cdaa3de8cd..4de1dbcd3ef64 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -315,7 +315,29 @@ static inline void copy_user_highpage(struct page *to, struct page *from,
>
>  #endif
>
> +#ifndef __HAVE_ARCH_COPY_HIGHPAGE
> +
> +static inline void copy_highpage(struct page *to, struct page *from)
> +{
> +       char *vfrom, *vto;
> +
> +       vfrom = kmap_local_page(from);
> +       vto = kmap_local_page(to);
> +       copy_page(vto, vfrom);
> +       kmsan_copy_page_meta(to, from);
> +       kunmap_local(vto);
> +       kunmap_local(vfrom);
> +}
> +
> +#endif
> +
>  #ifdef copy_mc_to_kernel
> +/*
> + * If architecture supports machine check exception handling, define the
> + * #MC versions of copy_user_highpage and copy_highpage. They copy a memory
> + * page with #MC in source page (@from) handled, and return the number
> + * of bytes not copied if there was a #MC, otherwise 0 for success.
> + */
>  static inline int copy_mc_user_highpage(struct page *to, struct page *from,
>                                         unsigned long vaddr, struct vm_area_struct *vma)
>  {
> @@ -332,29 +354,35 @@ static inline int copy_mc_user_highpage(struct page *to, struct page *from,
>
>         return ret;
>  }
> -#else
> -static inline int copy_mc_user_highpage(struct page *to, struct page *from,
> -                                       unsigned long vaddr, struct vm_area_struct *vma)
> -{
> -       copy_user_highpage(to, from, vaddr, vma);
> -       return 0;
> -}
> -#endif
>
> -#ifndef __HAVE_ARCH_COPY_HIGHPAGE
> -
> -static inline void copy_highpage(struct page *to, struct page *from)
> +static inline int copy_mc_highpage(struct page *to, struct page *from)
>  {
> +       unsigned long ret;
>         char *vfrom, *vto;
>
>         vfrom = kmap_local_page(from);
>         vto = kmap_local_page(to);
> -       copy_page(vto, vfrom);
> -       kmsan_copy_page_meta(to, from);
> +       ret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);
> +       if (!ret)
> +               kmsan_copy_page_meta(to, from);
>         kunmap_local(vto);
>         kunmap_local(vfrom);
> +
> +       return ret;
> +}
> +#else
> +static inline int copy_mc_user_highpage(struct page *to, struct page *from,
> +                                       unsigned long vaddr, struct vm_area_struct *vma)
> +{
> +       copy_user_highpage(to, from, vaddr, vma);
> +       return 0;
>  }
>
> +static inline int copy_mc_highpage(struct page *to, struct page *from)
> +{
> +       copy_highpage(to, from);
> +       return 0;
> +}
>  #endif
>
>  static inline void memcpy_page(struct page *dst_page, size_t dst_off,
> --
> 2.40.0.348.gf938b09366-goog
>

[PATCH v11 3/3] mm/khugepaged: recover from poisoned file-backed memory

[Jiaqi Yan]

Make collapse_file roll back when copying pages failed. More concretely:
- extract copying operations into a separate loop
- postpone the updates for nr_none until both scanning and copying
  succeeded
- postpone joining small xarray entries until both scanning and copying
  succeeded
- postpone the update operations to NR_XXX_THPS until both scanning and
  copying succeeded
- for non-SHMEM file, roll back filemap_nr_thps_inc if scan succeeded but
  copying failed

Tested manually:
0. Enable khugepaged on system under test. Mount tmpfs at /mnt/ramdisk.
1. Start a two-thread application. Each thread allocates a chunk of
   non-huge memory buffer from /mnt/ramdisk.
2. Pick 4 random buffer address (2 in each thread) and inject
   uncorrectable memory errors at physical addresses.
3. Signal both threads to make their memory buffer collapsible, i.e.
   calling madvise(MADV_HUGEPAGE).
4. Wait and then check kernel log: khugepaged is able to recover from
   poisoned pages by skipping them.
5. Signal both threads to inspect their buffer contents and make sure no
   data corruption.

Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
---
 mm/khugepaged.c | 86 +++++++++++++++++++++++++++++++------------------
 1 file changed, 54 insertions(+), 32 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index bef68286345c8..38c1655ce0a9e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1874,6 +1874,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 {
 	struct address_space *mapping = file->f_mapping;
 	struct page *hpage;
+	struct page *page;
+	struct page *tmp;
+	struct folio *folio;
 	pgoff_t index = 0, end = start + HPAGE_PMD_NR;
 	LIST_HEAD(pagelist);
 	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
@@ -1918,8 +1921,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 
 	xas_set(&xas, start);
 	for (index = start; index < end; index++) {
-		struct page *page = xas_next(&xas);
-		struct folio *folio;
+		page = xas_next(&xas);
 
 		VM_BUG_ON(index != xas.xa_index);
 		if (is_shmem) {
@@ -2099,12 +2101,8 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 		put_page(page);
 		goto xa_unlocked;
 	}
-	nr = thp_nr_pages(hpage);
 
-	if (is_shmem)
-		__mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
-	else {
-		__mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
+	if (!is_shmem) {
 		filemap_nr_thps_inc(mapping);
 		/*
 		 * Paired with smp_mb() in do_dentry_open() to ensure
@@ -2115,21 +2113,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 		smp_mb();
 		if (inode_is_open_for_write(mapping->host)) {
 			result = SCAN_FAIL;
-			__mod_lruvec_page_state(hpage, NR_FILE_THPS, -nr);
 			filemap_nr_thps_dec(mapping);
-			goto xa_locked;
 		}
 	}
-
-	if (nr_none) {
-		__mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
-		/* nr_none is always 0 for non-shmem. */
-		__mod_lruvec_page_state(hpage, NR_SHMEM, nr_none);
-	}
-
-	/* Join all the small entries into a single multi-index entry */
-	xas_set_order(&xas, start, HPAGE_PMD_ORDER);
-	xas_store(&xas, hpage);
 xa_locked:
 	xas_unlock_irq(&xas);
 xa_unlocked:
@@ -2142,21 +2128,36 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	try_to_unmap_flush();
 
 	if (result == SCAN_SUCCEED) {
-		struct page *page, *tmp;
-		struct folio *folio;
-
 		/*
 		 * Replacing old pages with new one has succeeded, now we
-		 * need to copy the content and free the old pages.
+		 * attempt to copy the contents.
 		 */
 		index = start;
-		list_for_each_entry_safe(page, tmp, &pagelist, lru) {
+		list_for_each_entry(page, &pagelist, lru) {
 			while (index < page->index) {
 				clear_highpage(hpage + (index % HPAGE_PMD_NR));
 				index++;
 			}
-			copy_highpage(hpage + (page->index % HPAGE_PMD_NR),
-				      page);
+			if (copy_mc_highpage(hpage + (page->index % HPAGE_PMD_NR),
+					     page) > 0) {
+				result = SCAN_COPY_MC;
+				break;
+			}
+			index++;
+		}
+		while (result == SCAN_SUCCEED && index < end) {
+			clear_highpage(hpage + (index % HPAGE_PMD_NR));
+			index++;
+		}
+	}
+
+	nr = thp_nr_pages(hpage);
+	if (result == SCAN_SUCCEED) {
+		/*
+		 * Copying old pages to huge one has succeeded, now we
+		 * need to free the old pages.
+		 */
+		list_for_each_entry_safe(page, tmp, &pagelist, lru) {
 			list_del(&page->lru);
 			page->mapping = NULL;
 			page_ref_unfreeze(page, 1);
@@ -2164,12 +2165,23 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 			ClearPageUnevictable(page);
 			unlock_page(page);
 			put_page(page);
-			index++;
 		}
-		while (index < end) {
-			clear_highpage(hpage + (index % HPAGE_PMD_NR));
-			index++;
+
+		xas_lock_irq(&xas);
+		if (is_shmem)
+			__mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
+		else
+			__mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
+
+		if (nr_none) {
+			__mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
+			/* nr_none is always 0 for non-shmem. */
+			__mod_lruvec_page_state(hpage, NR_SHMEM, nr_none);
 		}
+		/* Join all the small entries into a single multi-index entry. */
+		xas_set_order(&xas, start, HPAGE_PMD_ORDER);
+		xas_store(&xas, hpage);
+		xas_unlock_irq(&xas);
 
 		folio = page_folio(hpage);
 		folio_mark_uptodate(folio);
@@ -2187,8 +2199,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 		unlock_page(hpage);
 		hpage = NULL;
 	} else {
-		struct page *page;
-
 		/* Something went wrong: roll back page cache changes */
 		xas_lock_irq(&xas);
 		if (nr_none) {
@@ -2222,6 +2232,18 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 			xas_lock_irq(&xas);
 		}
 		VM_BUG_ON(nr_none);
+		/*
+		 * Undo the updates of filemap_nr_thps_inc for non-SHMEM
+		 * file only. This undo is not needed unless failure is
+		 * due to SCAN_COPY_MC.
+		 *
+		 * Paired with smp_mb() in do_dentry_open() to ensure the
+		 * update to nr_thps is visible.
+		 */
+		smp_mb();
+		if (!is_shmem && result == SCAN_COPY_MC)
+			filemap_nr_thps_dec(mapping);
+
 		xas_unlock_irq(&xas);
 
 		hpage->mapping = NULL;
-- 
2.40.0.348.gf938b09366-goog

Re: [PATCH v11 3/3] mm/khugepaged: recover from poisoned file-backed memory

[Yang Shi]

On Mon, Mar 27, 2023 at 2:16 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
>
> Make collapse_file roll back when copying pages failed. More concretely:
> - extract copying operations into a separate loop
> - postpone the updates for nr_none until both scanning and copying
>   succeeded
> - postpone joining small xarray entries until both scanning and copying
>   succeeded
> - postpone the update operations to NR_XXX_THPS until both scanning and
>   copying succeeded
> - for non-SHMEM file, roll back filemap_nr_thps_inc if scan succeeded but
>   copying failed
>
> Tested manually:
> 0. Enable khugepaged on system under test. Mount tmpfs at /mnt/ramdisk.
> 1. Start a two-thread application. Each thread allocates a chunk of
>    non-huge memory buffer from /mnt/ramdisk.
> 2. Pick 4 random buffer address (2 in each thread) and inject
>    uncorrectable memory errors at physical addresses.
> 3. Signal both threads to make their memory buffer collapsible, i.e.
>    calling madvise(MADV_HUGEPAGE).
> 4. Wait and then check kernel log: khugepaged is able to recover from
>    poisoned pages by skipping them.
> 5. Signal both threads to inspect their buffer contents and make sure no
>    data corruption.
>
> Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>

Reviewed-by: Yang Shi <shy828301@gmail.com>

A nit below:

> ---
>  mm/khugepaged.c | 86 +++++++++++++++++++++++++++++++------------------
>  1 file changed, 54 insertions(+), 32 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index bef68286345c8..38c1655ce0a9e 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1874,6 +1874,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>  {
>         struct address_space *mapping = file->f_mapping;
>         struct page *hpage;
> +       struct page *page;
> +       struct page *tmp;
> +       struct folio *folio;
>         pgoff_t index = 0, end = start + HPAGE_PMD_NR;
>         LIST_HEAD(pagelist);
>         XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
> @@ -1918,8 +1921,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>
>         xas_set(&xas, start);
>         for (index = start; index < end; index++) {
> -               struct page *page = xas_next(&xas);
> -               struct folio *folio;
> +               page = xas_next(&xas);
>
>                 VM_BUG_ON(index != xas.xa_index);
>                 if (is_shmem) {
> @@ -2099,12 +2101,8 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>                 put_page(page);
>                 goto xa_unlocked;
>         }
> -       nr = thp_nr_pages(hpage);
>
> -       if (is_shmem)
> -               __mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
> -       else {
> -               __mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
> +       if (!is_shmem) {
>                 filemap_nr_thps_inc(mapping);
>                 /*
>                  * Paired with smp_mb() in do_dentry_open() to ensure
> @@ -2115,21 +2113,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>                 smp_mb();
>                 if (inode_is_open_for_write(mapping->host)) {
>                         result = SCAN_FAIL;
> -                       __mod_lruvec_page_state(hpage, NR_FILE_THPS, -nr);
>                         filemap_nr_thps_dec(mapping);
> -                       goto xa_locked;
>                 }
>         }
> -
> -       if (nr_none) {
> -               __mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
> -               /* nr_none is always 0 for non-shmem. */
> -               __mod_lruvec_page_state(hpage, NR_SHMEM, nr_none);
> -       }
> -
> -       /* Join all the small entries into a single multi-index entry */
> -       xas_set_order(&xas, start, HPAGE_PMD_ORDER);
> -       xas_store(&xas, hpage);
>  xa_locked:
>         xas_unlock_irq(&xas);
>  xa_unlocked:
> @@ -2142,21 +2128,36 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>         try_to_unmap_flush();
>
>         if (result == SCAN_SUCCEED) {
> -               struct page *page, *tmp;
> -               struct folio *folio;
> -
>                 /*
>                  * Replacing old pages with new one has succeeded, now we
> -                * need to copy the content and free the old pages.
> +                * attempt to copy the contents.
>                  */
>                 index = start;
> -               list_for_each_entry_safe(page, tmp, &pagelist, lru) {
> +               list_for_each_entry(page, &pagelist, lru) {
>                         while (index < page->index) {
>                                 clear_highpage(hpage + (index % HPAGE_PMD_NR));
>                                 index++;
>                         }
> -                       copy_highpage(hpage + (page->index % HPAGE_PMD_NR),
> -                                     page);
> +                       if (copy_mc_highpage(hpage + (page->index % HPAGE_PMD_NR),
> +                                            page) > 0) {
> +                               result = SCAN_COPY_MC;
> +                               break;
> +                       }
> +                       index++;
> +               }
> +               while (result == SCAN_SUCCEED && index < end) {
> +                       clear_highpage(hpage + (index % HPAGE_PMD_NR));
> +                       index++;
> +               }
> +       }
> +
> +       nr = thp_nr_pages(hpage);
> +       if (result == SCAN_SUCCEED) {
> +               /*
> +                * Copying old pages to huge one has succeeded, now we
> +                * need to free the old pages.
> +                */
> +               list_for_each_entry_safe(page, tmp, &pagelist, lru) {
>                         list_del(&page->lru);
>                         page->mapping = NULL;
>                         page_ref_unfreeze(page, 1);
> @@ -2164,12 +2165,23 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>                         ClearPageUnevictable(page);
>                         unlock_page(page);
>                         put_page(page);
> -                       index++;
>                 }
> -               while (index < end) {
> -                       clear_highpage(hpage + (index % HPAGE_PMD_NR));
> -                       index++;
> +
> +               xas_lock_irq(&xas);
> +               if (is_shmem)
> +                       __mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
> +               else
> +                       __mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
> +
> +               if (nr_none) {
> +                       __mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
> +                       /* nr_none is always 0 for non-shmem. */
> +                       __mod_lruvec_page_state(hpage, NR_SHMEM, nr_none);
>                 }
> +               /* Join all the small entries into a single multi-index entry. */
> +               xas_set_order(&xas, start, HPAGE_PMD_ORDER);
> +               xas_store(&xas, hpage);
> +               xas_unlock_irq(&xas);
>
>                 folio = page_folio(hpage);
>                 folio_mark_uptodate(folio);
> @@ -2187,8 +2199,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>                 unlock_page(hpage);
>                 hpage = NULL;
>         } else {
> -               struct page *page;
> -
>                 /* Something went wrong: roll back page cache changes */
>                 xas_lock_irq(&xas);
>                 if (nr_none) {
> @@ -2222,6 +2232,18 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>                         xas_lock_irq(&xas);
>                 }
>                 VM_BUG_ON(nr_none);
> +               /*
> +                * Undo the updates of filemap_nr_thps_inc for non-SHMEM
> +                * file only. This undo is not needed unless failure is
> +                * due to SCAN_COPY_MC.
> +                *
> +                * Paired with smp_mb() in do_dentry_open() to ensure the
> +                * update to nr_thps is visible.
> +                */
> +               smp_mb();
> +               if (!is_shmem && result == SCAN_COPY_MC)
> +                       filemap_nr_thps_dec(mapping);

I think the memory barrier should be after the dec.

> +
>                 xas_unlock_irq(&xas);
>
>                 hpage->mapping = NULL;
> --
> 2.40.0.348.gf938b09366-goog
>

Re: [PATCH v11 3/3] mm/khugepaged: recover from poisoned file-backed memory

[Jiaqi Yan]

On Tue, Mar 28, 2023 at 9:02 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Mon, Mar 27, 2023 at 2:16 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
> >
> > Make collapse_file roll back when copying pages failed. More concretely:
> > - extract copying operations into a separate loop
> > - postpone the updates for nr_none until both scanning and copying
> >   succeeded
> > - postpone joining small xarray entries until both scanning and copying
> >   succeeded
> > - postpone the update operations to NR_XXX_THPS until both scanning and
> >   copying succeeded
> > - for non-SHMEM file, roll back filemap_nr_thps_inc if scan succeeded but
> >   copying failed
> >
> > Tested manually:
> > 0. Enable khugepaged on system under test. Mount tmpfs at /mnt/ramdisk.
> > 1. Start a two-thread application. Each thread allocates a chunk of
> >    non-huge memory buffer from /mnt/ramdisk.
> > 2. Pick 4 random buffer address (2 in each thread) and inject
> >    uncorrectable memory errors at physical addresses.
> > 3. Signal both threads to make their memory buffer collapsible, i.e.
> >    calling madvise(MADV_HUGEPAGE).
> > 4. Wait and then check kernel log: khugepaged is able to recover from
> >    poisoned pages by skipping them.
> > 5. Signal both threads to inspect their buffer contents and make sure no
> >    data corruption.
> >
> > Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
>
> Reviewed-by: Yang Shi <shy828301@gmail.com>
>
> A nit below:
>
> > ---
> >  mm/khugepaged.c | 86 +++++++++++++++++++++++++++++++------------------
> >  1 file changed, 54 insertions(+), 32 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index bef68286345c8..38c1655ce0a9e 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1874,6 +1874,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >  {
> >         struct address_space *mapping = file->f_mapping;
> >         struct page *hpage;
> > +       struct page *page;
> > +       struct page *tmp;
> > +       struct folio *folio;
> >         pgoff_t index = 0, end = start + HPAGE_PMD_NR;
> >         LIST_HEAD(pagelist);
> >         XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
> > @@ -1918,8 +1921,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >
> >         xas_set(&xas, start);
> >         for (index = start; index < end; index++) {
> > -               struct page *page = xas_next(&xas);
> > -               struct folio *folio;
> > +               page = xas_next(&xas);
> >
> >                 VM_BUG_ON(index != xas.xa_index);
> >                 if (is_shmem) {
> > @@ -2099,12 +2101,8 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >                 put_page(page);
> >                 goto xa_unlocked;
> >         }
> > -       nr = thp_nr_pages(hpage);
> >
> > -       if (is_shmem)
> > -               __mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
> > -       else {
> > -               __mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
> > +       if (!is_shmem) {
> >                 filemap_nr_thps_inc(mapping);
> >                 /*
> >                  * Paired with smp_mb() in do_dentry_open() to ensure
> > @@ -2115,21 +2113,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >                 smp_mb();
> >                 if (inode_is_open_for_write(mapping->host)) {
> >                         result = SCAN_FAIL;
> > -                       __mod_lruvec_page_state(hpage, NR_FILE_THPS, -nr);
> >                         filemap_nr_thps_dec(mapping);
> > -                       goto xa_locked;
> >                 }
> >         }
> > -
> > -       if (nr_none) {
> > -               __mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
> > -               /* nr_none is always 0 for non-shmem. */
> > -               __mod_lruvec_page_state(hpage, NR_SHMEM, nr_none);
> > -       }
> > -
> > -       /* Join all the small entries into a single multi-index entry */
> > -       xas_set_order(&xas, start, HPAGE_PMD_ORDER);
> > -       xas_store(&xas, hpage);
> >  xa_locked:
> >         xas_unlock_irq(&xas);
> >  xa_unlocked:
> > @@ -2142,21 +2128,36 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >         try_to_unmap_flush();
> >
> >         if (result == SCAN_SUCCEED) {
> > -               struct page *page, *tmp;
> > -               struct folio *folio;
> > -
> >                 /*
> >                  * Replacing old pages with new one has succeeded, now we
> > -                * need to copy the content and free the old pages.
> > +                * attempt to copy the contents.
> >                  */
> >                 index = start;
> > -               list_for_each_entry_safe(page, tmp, &pagelist, lru) {
> > +               list_for_each_entry(page, &pagelist, lru) {
> >                         while (index < page->index) {
> >                                 clear_highpage(hpage + (index % HPAGE_PMD_NR));
> >                                 index++;
> >                         }
> > -                       copy_highpage(hpage + (page->index % HPAGE_PMD_NR),
> > -                                     page);
> > +                       if (copy_mc_highpage(hpage + (page->index % HPAGE_PMD_NR),
> > +                                            page) > 0) {
> > +                               result = SCAN_COPY_MC;
> > +                               break;
> > +                       }
> > +                       index++;
> > +               }
> > +               while (result == SCAN_SUCCEED && index < end) {
> > +                       clear_highpage(hpage + (index % HPAGE_PMD_NR));
> > +                       index++;
> > +               }
> > +       }
> > +
> > +       nr = thp_nr_pages(hpage);
> > +       if (result == SCAN_SUCCEED) {
> > +               /*
> > +                * Copying old pages to huge one has succeeded, now we
> > +                * need to free the old pages.
> > +                */
> > +               list_for_each_entry_safe(page, tmp, &pagelist, lru) {
> >                         list_del(&page->lru);
> >                         page->mapping = NULL;
> >                         page_ref_unfreeze(page, 1);
> > @@ -2164,12 +2165,23 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >                         ClearPageUnevictable(page);
> >                         unlock_page(page);
> >                         put_page(page);
> > -                       index++;
> >                 }
> > -               while (index < end) {
> > -                       clear_highpage(hpage + (index % HPAGE_PMD_NR));
> > -                       index++;
> > +
> > +               xas_lock_irq(&xas);
> > +               if (is_shmem)
> > +                       __mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
> > +               else
> > +                       __mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
> > +
> > +               if (nr_none) {
> > +                       __mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
> > +                       /* nr_none is always 0 for non-shmem. */
> > +                       __mod_lruvec_page_state(hpage, NR_SHMEM, nr_none);
> >                 }
> > +               /* Join all the small entries into a single multi-index entry. */
> > +               xas_set_order(&xas, start, HPAGE_PMD_ORDER);
> > +               xas_store(&xas, hpage);
> > +               xas_unlock_irq(&xas);
> >
> >                 folio = page_folio(hpage);
> >                 folio_mark_uptodate(folio);
> > @@ -2187,8 +2199,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >                 unlock_page(hpage);
> >                 hpage = NULL;
> >         } else {
> > -               struct page *page;
> > -
> >                 /* Something went wrong: roll back page cache changes */
> >                 xas_lock_irq(&xas);
> >                 if (nr_none) {
> > @@ -2222,6 +2232,18 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >                         xas_lock_irq(&xas);
> >                 }
> >                 VM_BUG_ON(nr_none);
> > +               /*
> > +                * Undo the updates of filemap_nr_thps_inc for non-SHMEM
> > +                * file only. This undo is not needed unless failure is
> > +                * due to SCAN_COPY_MC.
> > +                *
> > +                * Paired with smp_mb() in do_dentry_open() to ensure the
> > +                * update to nr_thps is visible.
> > +                */
> > +               smp_mb();
> > +               if (!is_shmem && result == SCAN_COPY_MC)
> > +                       filemap_nr_thps_dec(mapping);
>
> I think the memory barrier should be after the dec.

Ah, will move into the if block and put after filemap_nr_thps_dec.

>
> > +
> >                 xas_unlock_irq(&xas);
> >
> >                 hpage->mapping = NULL;
> > --
> > 2.40.0.348.gf938b09366-goog
> >