Re: [PATCH v3 1/2] mm: khugepaged: recover from poisoned anonymous memory

[Jiaqi Yan <jiaqiyan@google.com>]

On Tue, May 24, 2022 at 11:41 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Mon, May 23, 2022 at 7:54 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
> >
> > Make __collapse_huge_page_copy return whether
> > collapsing/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. Roll back the page table and page states
> > when copying failed:
> > 1) re-establish the PTEs-to-PMD connection.
> > 2) release pages back to their LRU list.
> >
> > Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
> > ---
> >  include/linux/highmem.h            |  19 ++++
> >  include/trace/events/huge_memory.h |  27 +++++-
> >  mm/khugepaged.c                    | 146 ++++++++++++++++++++++-------
> >  3 files changed, 158 insertions(+), 34 deletions(-)
> >
> > diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> > index 39bb9b47fa9cd..0ccb1e92c4b06 100644
> > --- a/include/linux/highmem.h
> > +++ b/include/linux/highmem.h
> > @@ -298,6 +298,25 @@ static inline void copy_highpage(struct page *to, struct page *from)
> >
> >  #endif
> >
> > +/*
> > + * Machine check exception handled version of copy_highpage.
> > + * Return true if copying page content failed; otherwise false.
> > + * Note handling #MC requires arch opt-in.
> > + */
> > +static inline bool copy_highpage_mc(struct page *to, struct page *from)
> > +{
> > +       char *vfrom, *vto;
> > +       unsigned long ret;
> > +
> > +       vfrom = kmap_local_page(from);
> > +       vto = kmap_local_page(to);
> > +       ret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);
> > +       kunmap_local(vto);
> > +       kunmap_local(vfrom);
> > +
> > +       return ret > 0;
> > +}
> > +
> >  static inline void memcpy_page(struct page *dst_page, size_t dst_off,
> >                                struct page *src_page, size_t src_off,
> >                                size_t len)
> > diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> > index 4fdb14a81108b..7e02c0aa2f728 100644
> > --- a/include/trace/events/huge_memory.h
> > +++ b/include/trace/events/huge_memory.h
> > @@ -34,7 +34,8 @@
> >         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
> > @@ -167,5 +168,29 @@ TRACE_EVENT(mm_collapse_huge_page_swapin,
> >                 __entry->ret)
> >  );
> >
> > +TRACE_EVENT(mm_collapse_huge_page_copy,
>
> You may misunderstand my point. I don't mean we need a new tracepoint,
> IMHO trace_mm_collapse_huge_page() is good enough. When copy is failed
> you do have "result = SCAN_COPY_MC" and the result will be consumed by
> trace_mm_collapse_huge_page(). You just need have trace interpret the
> new result as the above section does.

I see. To make sure we are on the same page, I think we only need to add:

    EMe(SCAN_COPY_MC, "copy_poisoned_page")

I don't think we need to add a 'copied' arg in mm_collapse_huge_page() as
"copy_poisoned_page" is the only reason copied=false.

My intention to add a new tracepoint is for showing the poisoned PFN but I feel
you don't think this is very important information w.r.t khugepaged.
The important
thing is we skipped the poisoned page(s).

>
> > +
> > +       TP_PROTO(struct page *start, struct page *poisoned, int status),
> > +
> > +       TP_ARGS(start, poisoned, status),
> > +
> > +       TP_STRUCT__entry(
> > +               __field(unsigned long, start_pfn)
> > +               __field(unsigned long, poisoned_pfn)
> > +               __field(int, status)
> > +       ),
> > +
> > +       TP_fast_assign(
> > +               __entry->start_pfn = start ? page_to_pfn(start) : -1;
> > +               __entry->poisoned_pfn = poisoned ? page_to_pfn(poisoned) : -1;
> > +               __entry->status = status;
> > +       ),
> > +
> > +       TP_printk("start_pfn=0x%lx, poisoned_pfn=0x%lx, status=%s",
> > +               __entry->start_pfn,
> > +               __entry->poisoned_pfn,
> > +               __print_symbolic(__entry->status, SCAN_STATUS))
> > +);
> > +
> >  #endif /* __HUGE_MEMORY_H */
> >  #include <trace/define_trace.h>
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 131492fd1148b..1b08e31ba081a 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -52,6 +52,7 @@ enum scan_result {
> >         SCAN_CGROUP_CHARGE_FAIL,
> >         SCAN_TRUNCATED,
> >         SCAN_PAGE_HAS_PRIVATE,
> > +       SCAN_COPY_MC,
> >  };
> >
> >  #define CREATE_TRACE_POINTS
> > @@ -739,44 +740,104 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >         return 0;
> >  }
> >
> > -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)
> > +/*
> > + * __collapse_huge_page_copy - attempts to copy memory contents from normal
> > + * pages to a hugepage. Cleanup the normal pages if copying succeeds;
> > + * otherwise restore the original page table and release isolated normal pages.
> > + * Returns true if copying succeeds, otherwise returns false.
> > + *
> > + * @pte: starting of the PTEs to copy from
> > + * @page: the new hugepage to copy contents to
> > + * @pmd: pointer to the new hugepage's PMD
> > + * @rollback: the orginal normal pages' PMD
>
> s/orginal/original
>
> > + * @address: starting address to copy
> > + * @pte_ptl: lock on normal pages' PTEs
> > + * @compound_pagelist: list that stores compound pages
> > + */
> > +static bool __collapse_huge_page_copy(pte_t *pte,
> > +                               struct page *page,
> > +                               pmd_t *pmd,
> > +                               pmd_t rollback,
> > +                               struct vm_area_struct *vma,
> > +                               unsigned long address,
> > +                               spinlock_t *pte_ptl,
> > +                               struct list_head *compound_pagelist)
> >  {
> >         struct page *src_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;
> > +       unsigned long _address;
> > +       spinlock_t *pmd_ptl;
> > +       bool copy_succeeded = true;
> >
> > -               if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > +       /*
> > +        * 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);
> > -                       add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> > -                       if (is_zero_pfn(pte_pfn(pteval))) {
> > -                               /*
> > -                                * ptl mostly unnecessary.
> > -                                */
> > -                               spin_lock(ptl);
> > -                               ptep_clear(vma->vm_mm, address, _pte);
> > -                               spin_unlock(ptl);
> > +               else {
> > +                       src_page = pte_page(pteval);
> > +                       if (copy_highpage_mc(page, src_page)) {
> > +                               copy_succeeded = false;
> > +                               trace_mm_collapse_huge_page_copy(pte_page(*pte),
> > +                                               src_page, SCAN_COPY_MC);
>
> As aforementioned I don't think we need a new trace point.
>
> > +                               break;
> > +                       }
> > +               }
> > +       }
> > +
> > +       if (copy_succeeded)
> > +               trace_mm_collapse_huge_page_copy(pte_page(*pte),
> > +                               NULL, SCAN_SUCCEED);
> > +       else {
> > +               /*
> > +                * Copying failed, re-establish the regular PMD that points to
> > +                * the regular page table. 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(rollback));
> > +               spin_unlock(pmd_ptl);
> > +       }
> > +
> > +       for (_pte = pte, _address = address; _pte < pte + HPAGE_PMD_NR;
> > +                       _pte++, _address += PAGE_SIZE) {
> > +               pteval = *_pte;
> > +               if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > +                       if (copy_succeeded) {
> > +                               add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> > +                               if (is_zero_pfn(pte_pfn(pteval))) {
> > +                                       /*
> > +                                        * ptl mostly unnecessary.
> > +                                        */
> > +                                       spin_lock(pte_ptl);
> > +                                       pte_clear(vma->vm_mm, _address, _pte);
> > +                                       spin_unlock(pte_ptl);
> > +                               }
> >                         }
> >                 } else {
> >                         src_page = pte_page(pteval);
> > -                       copy_user_highpage(page, src_page, address, vma);
> >                         if (!PageCompound(src_page))
> >                                 release_pte_page(src_page);
> > -                       /*
> > -                        * ptl mostly unnecessary, but preempt has to
> > -                        * be disabled to update the per-cpu stats
> > -                        * inside page_remove_rmap().
> > -                        */
> > -                       spin_lock(ptl);
> > -                       ptep_clear(vma->vm_mm, address, _pte);
> > -                       page_remove_rmap(src_page, false);
> > -                       spin_unlock(ptl);
> > -                       free_page_and_swap_cache(src_page);
> > +
> > +                       if (copy_succeeded) {
> > +                               /*
> > +                                * ptl mostly unnecessary, but preempt has to
> > +                                * be disabled to update the per-cpu stats
> > +                                * inside page_remove_rmap().
> > +                                */
> > +                               spin_lock(pte_ptl);
> > +                               pte_clear(vma->vm_mm, _address, _pte);
> > +                               page_remove_rmap(src_page, false);
> > +                               spin_unlock(pte_ptl);
> > +                               free_page_and_swap_cache(src_page);
> > +                       }
> >                 }
> >         }
> >
> > @@ -784,6 +845,8 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> >                 list_del(&src_page->lru);
> >                 release_pte_page(src_page);
> >         }
> > +
> > +       return copy_succeeded;
> >  }
> >
> >  static void khugepaged_alloc_sleep(void)
> > @@ -1066,6 +1129,7 @@ static void collapse_huge_page(struct mm_struct *mm,
> >         struct vm_area_struct *vma;
> >         struct mmu_notifier_range range;
> >         gfp_t gfp;
> > +       bool copied = false;
> >
> >         VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> >
> > @@ -1177,9 +1241,13 @@ static void collapse_huge_page(struct mm_struct *mm,
> >          */
> >         anon_vma_unlock_write(vma->anon_vma);
> >
> > -       __collapse_huge_page_copy(pte, new_page, vma, address, pte_ptl,
> > -                       &compound_pagelist);
> > +       copied = __collapse_huge_page_copy(pte, new_page, pmd, _pmd,
> > +                       vma, address, pte_ptl, &compound_pagelist);
> >         pte_unmap(pte);
> > +       if (!copied) {
> > +               result = SCAN_COPY_MC;
> > +               goto out_up_write;
> > +       }
> >         /*
> >          * spin_lock() below is not the equivalent of smp_wmb(), but
> >          * the smp_wmb() inside __SetPageUptodate() can be reused to
> > @@ -1364,9 +1432,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> >         pte_unmap_unlock(pte, ptl);
> >         if (ret) {
> >                 node = khugepaged_find_target_node();
> > -               /* collapse_huge_page will return with the mmap_lock released */
> > -               collapse_huge_page(mm, address, hpage, node,
> > -                               referenced, unmapped);
> > +               /*
> > +                * collapse_huge_page will return with the mmap_r+w_lock released.
> > +                * It is uncertain if *hpage is NULL or not when collapse_huge_page
>
> Why is it uncertain?

By "uncertain" I mean *hpage may not be set to NULL near the end of
collapse_huge_page
if swapin/isolation/copying failed.

>
> > +                * returns, so keep ret=1 to jump to breakouterloop_mmap_lock
>
> "ret=1" means scan succeeded so mmap_lock is released. But this
> happens in the caller.
>
> > +                * in khugepaged_scan_mm_slot, then *hpage will be freed
> > +                * if collapse failed.
>
> It depends. For CONFIG_NUMA, it will be freed since next scan may need
> a page from a different node, but !CONFIG_NUMA may not free it, it may
> be reused IIUC. We do intend to remove this optimization, but it has
> not happened yet.

Yes, you are right if !CONFIG_NUMA, khugepaged_prealloc_page only
free *hpage when page_count(*hpage) > 1.

>
> TBH I don't think the comment is helpful here and actually confusing,
> I'd prefer to have it right before calling khugepaged_scan_pmd() if
> you would like to keep it.

I will remove the comment and revert this part.

>
> > +                */
> > +               collapse_huge_page(mm, address, hpage, node, referenced, unmapped);
> >         }
> >  out:
> >         trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
> > @@ -2168,6 +2241,13 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
> >                                 khugepaged_scan_file(mm, file, pgoff, hpage);
> >                                 fput(file);
> >                         } else {
> > +                               /*
> > +                                * mmap_read_lock is
> > +                                * 1) still held if scan failed;
> > +                                * 2) released if scan succeeded.
> > +                                * It is not affected by collapsing or copying
> > +                                * operations.
> > +                                */
> >                                 ret = khugepaged_scan_pmd(mm, vma,
> >                                                 khugepaged_scan.address,
> >                                                 hpage);
> > --
> > 2.36.1.124.g0e6072fb45-goog
> >