Re: [PATCH v12 0/3] Memory poison recovery in khugepaged collapsing

[Jiaqi Yan <jiaqiyan@google.com>]

On Tue, Apr 4, 2023 at 8:57 PM Yang Shi <shy828301@gmail.com> wrote:
>
> On Tue, Apr 4, 2023 at 11:44 AM Jiaqi Yan <jiaqiyan@google.com> wrote:
> >
> > Friendly ping for review :)
>
> Both I and Hugh already gave reviewed/acked for the previous version.
> Since there were just some minor changes so you could keep the
> reviewed/acked from the previous version.

Thanks Yang!
Andrew, is there still anything I need to do at this point (e.g.
resent V12 with reviewed/acked tags in commits)?
Or are you fine with this V12 to be merged?

>
> >
> > On Wed, Mar 29, 2023 at 8:11 AM Jiaqi Yan <jiaqiyan@google.com> wrote:
> >>
> >> 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
> >> =========
> >> v12 changes
> >> - Incorporate feedbacks from Shi Yang <shy828301@gmail.com>.
> >> - Drop unused pmd from __collapse_huge_page_copy_succeeded.
> >> - Drop unused address from __collapse_huge_page_copy_failed.
> >> - smp_mb() should be after filemap_nr_thps_dec.
> >> - This revision is rebased to mm-unstable at commit 9b175ce664d33
> >>   ("mm: move free_area_empty() to mm/internal.h")
> >>
> >> 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
> >>