Re: [PATCH v1 2/2] mm/memory-failure: avoid free HWPoison high-order folio

[Jiaqi Yan <jiaqiyan@google.com>]

On Sat, Nov 15, 2025 at 6:10 PM Zi Yan <ziy@nvidia.com> wrote:
>
> On 15 Nov 2025, at 20:47, Jiaqi Yan wrote:
>
> > At the end of dissolve_free_hugetlb_folio, when a free HugeTLB
> > folio becomes non-HugeTLB, it is released to buddy allocator
> > as a high-order folio, e.g. a folio that contains 262144 pages
> > if the folio was a 1G HugeTLB hugepage.
> >
> > This is problematic if the HugeTLB hugepage contained HWPoison
> > subpages. In that case, since buddy allocator does not check
> > HWPoison for non-zero-order folio, the raw HWPoison page can
> > be given out with its buddy page and be re-used by either
> > kernel or userspace.
> >
> > Memory failure recovery (MFR) in kernel does attempt to take
> > raw HWPoison page off buddy allocator after
> > dissolve_free_hugetlb_folio. However, there is always a time
> > window between freed to buddy allocator and taken off from
> > buddy allocator.
> >
> > One obvious way to avoid this problem is to add page sanity
> > checks in page allocate or free path. However, it is against
> > the past efforts to reduce sanity check overhead [1,2,3].
> >
> > Introduce hugetlb_free_hwpoison_folio to solve this problem.
> > The idea is, in case a HugeTLB folio for sure contains HWPoison
> > page(s), first split the non-HugeTLB high-order folio uniformly
> > into 0-order folios, then let healthy pages join the buddy
> > allocator while reject the HWPoison ones.
> >
> > [1] https://lore.kernel.org/linux-mm/1460711275-1130-15-git-send-email-mgorman@techsingularity.net/
> > [2] https://lore.kernel.org/linux-mm/1460711275-1130-16-git-send-email-mgorman@techsingularity.net/
> > [3] https://lore.kernel.org/all/20230216095131.17336-1-vbabka@suse.cz
> >
> > Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
> > ---
> >  include/linux/hugetlb.h |  4 ++++
> >  mm/hugetlb.c            |  8 ++++++--
> >  mm/memory-failure.c     | 43 +++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 53 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index 8e63e46b8e1f0..e1c334a7db2fe 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -870,8 +870,12 @@ int dissolve_free_hugetlb_folios(unsigned long start_pfn,
> >                                   unsigned long end_pfn);
> >
> >  #ifdef CONFIG_MEMORY_FAILURE
> > +extern void hugetlb_free_hwpoison_folio(struct folio *folio);
> >  extern void folio_clear_hugetlb_hwpoison(struct folio *folio);
> >  #else
> > +static inline void hugetlb_free_hwpoison_folio(struct folio *folio)
> > +{
> > +}
> >  static inline void folio_clear_hugetlb_hwpoison(struct folio *folio)
> >  {
> >  }
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 0455119716ec0..801ca1a14c0f0 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1596,6 +1596,7 @@ static void __update_and_free_hugetlb_folio(struct hstate *h,
> >                                               struct folio *folio)
> >  {
> >       bool clear_flag = folio_test_hugetlb_vmemmap_optimized(folio);
> > +     bool has_hwpoison = folio_test_hwpoison(folio);
> >
> >       if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> >               return;
> > @@ -1638,12 +1639,15 @@ static void __update_and_free_hugetlb_folio(struct hstate *h,
> >        * Move PageHWPoison flag from head page to the raw error pages,
> >        * which makes any healthy subpages reusable.
> >        */
> > -     if (unlikely(folio_test_hwpoison(folio)))
> > +     if (unlikely(has_hwpoison))
> >               folio_clear_hugetlb_hwpoison(folio);
> >
> >       folio_ref_unfreeze(folio, 1);
> >
> > -     hugetlb_free_folio(folio);
> > +     if (unlikely(has_hwpoison))
> > +             hugetlb_free_hwpoison_folio(folio);
> > +     else
> > +             hugetlb_free_folio(folio);
> >  }
> >
> >  /*
> > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > index 3edebb0cda30b..e6a9deba6292a 100644
> > --- a/mm/memory-failure.c
> > +++ b/mm/memory-failure.c
> > @@ -2002,6 +2002,49 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags,
> >       return ret;
> >  }
> >
> > +void hugetlb_free_hwpoison_folio(struct folio *folio)
> > +{
> > +     struct folio *curr, *next;
> > +     struct folio *end_folio = folio_next(folio);
> > +     int ret;
> > +
> > +     VM_WARN_ON_FOLIO(folio_ref_count(folio) != 1, folio);
> > +
> > +     ret = uniform_split_unmapped_folio_to_zero_order(folio);
>
> I realize that __split_unmapped_folio() is a wrong name and causes confusion.
> It should be __split_frozen_folio(), since when you look at its current
> call site, it is called after the folio is frozen. There probably
> should be a check in __split_unmapped_folio() to make sure the folio is frozen.
>
> Is it possible to change hugetlb_free_hwpoison_folio() so that it
> can be called before folio_ref_unfreeze(folio, 1)? In this way,
> __split_unmapped_folio() is called at frozen folios.
>
> You can add a preparation patch to rename __split_unmapped_folio() to
> __split_frozen_folio() and add
> VM_WARN_ON_ONCE_FOLIO(folio_ref_count(folio) != 0, folio) to the function.
>

FWIW, I am going to still follow your suggestion to improve code
healthiness or readability :)

> Thanks.

Thanks, Zi!

>
> > +     if (ret) {
> > +             /*
> > +              * In case of split failure, none of the pages in folio
> > +              * will be freed to buddy allocator.
> > +              */
> > +             pr_err("%#lx: failed to split free %d-order folio with HWPoison page(s): %d\n",
> > +                    folio_pfn(folio), folio_order(folio), ret);
> > +             return;
> > +     }
> > +
> > +     /* Expect 1st folio's refcount==1, and other's refcount==0. */
> > +     for (curr = folio; curr != end_folio; curr = next) {
> > +             next = folio_next(curr);
> > +
> > +             VM_WARN_ON_FOLIO(folio_order(curr), curr);
> > +
> > +             if (PageHWPoison(&curr->page)) {
> > +                     if (curr != folio)
> > +                             folio_ref_inc(curr);
> > +
> > +                     VM_WARN_ON_FOLIO(folio_ref_count(curr) != 1, curr);
> > +                     pr_warn("%#lx: prevented freeing HWPoison page\n",
> > +                             folio_pfn(curr));
> > +                     continue;
> > +             }
> > +
> > +             if (curr == folio)
> > +                     folio_ref_dec(curr);
> > +
> > +             VM_WARN_ON_FOLIO(folio_ref_count(curr), curr);
> > +             free_frozen_pages(&curr->page, folio_order(curr));
> > +     }
> > +}
> > +
> >  /*
> >   * Taking refcount of hugetlb pages needs extra care about race conditions
> >   * with basic operations like hugepage allocation/free/demotion.
> > --
> > 2.52.0.rc1.455.g30608eb744-goog
>
>
> --
> Best Regards,
> Yan, Zi