Re: [PATCH v3 2/3] mm/page_alloc: only free healthy pages in high-order has_hwpoisoned folio

[Jiaqi Yan <jiaqiyan@google.com>]

On Mon, Jan 12, 2026 at 9:39 PM Harry Yoo <harry.yoo@oracle.com> wrote:
>
> On Mon, Jan 12, 2026 at 12:49:22AM +0000, Jiaqi Yan wrote:
> > At the end of dissolve_free_hugetlb_folio(), a free HugeTLB folio
> > becomes non-HugeTLB, and 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 dissolve_free_hugetlb_folio() frees a HWPoison
> > high-order folio to buddy allocator and MFR takes HWPoison
> > raw page off buddy allocator.
>
> I wonder if this is something we want to backport to -stable.
>
> > 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 free_has_hwpoisoned() to only free the healthy pages
> > and to exclude the HWPoison ones in the high-order folio.
> > The idea is to iterate through the sub-pages of the folio to
> > identify contiguous ranges of healthy pages. Instead of freeing
> > pages one by one, decompose healthy ranges into the largest
> > possible blocks having different orders. Every block meets the
> > requirements to be freed via __free_one_page().
> >
> > free_has_hwpoisoned() has linear time complexity wrt the number
> > of pages in the folio. While the power-of-two decomposition
> > ensures that the number of calls to the buddy allocator is
> > logarithmic for each contiguous healthy range, the mandatory
> > linear scan of pages to identify PageHWPoison() defines the
> > overall time complexity. For a 1G hugepage having several
> > HWPoison pages, free_has_hwpoisoned() takes around 2ms on
> > average.
> >
> > Since free_has_hwpoisoned() has nontrivial overhead, it is
> > wrapped inside free_pages_prepare_has_hwpoisoned() and done
> > only PG_has_hwpoisoned indicates HWPoison page exists and
> > after free_pages_prepare() succeeded.
> >
> > [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>
> >
> > ---
> >  mm/page_alloc.c | 157 +++++++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 154 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 822e05f1a9646..9393589118604 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2923,6 +2928,152 @@ static bool free_frozen_page_commit(struct zone *zone,
> >       return ret;
> >  }
>
> From correctness point of view I think it looks good to me.

Thanks, Harry!

> Let's see what the page allocator folks say.
>
> A few nits below.
>
> > +static bool compound_has_hwpoisoned(struct page *page, unsigned int order)
> > +{
> > +     if (order == 0 || !PageCompound(page))
> > +             return false;
>
> nit: since order-0 compound page is not a thing,
> !PageCompound(page) check should cover order == 0 case.

ack, will simplify to something like PageCompound && folio_test_has_hwpoisoned.

>
> > +     return folio_test_has_hwpoisoned(page_folio(page));
> > +}
> > +
> > +/*
> > + * Do free_has_hwpoisoned() when needed after free_pages_prepare().
> > + * Returns
> > + * - true: free_pages_prepare() is good and caller can proceed freeing.
> > + * - false: caller should not free pages for one of the two reasons:
> > + *   1. free_pages_prepare() failed so it is not safe to proceed freeing.
> > + *   2. this is a compound page having some HWPoison pages, and healthy
> > + *      pages are already safely freed.
> > + */
> > +static bool free_pages_prepare_has_hwpoisoned(struct page *page,
> > +                                           unsigned int order,
> > +                                           fpi_t fpi_flags)
>
> nit: Hope we'll come up with a better name than
> free_pages_prepare_has_poisoned(), but I don't have any better
> suggestion... :)
>
> And I hope somebody familiar with compaction (as compaction_free() calls
> free_pages_prepare() and ignores its return value) could confirm that
> it is safe to do a compound_has_hwpoisoned() check and, when it returns
> true, call free_has_hwpoisoned() in free_pages_prepare(),
> so that we won't need a separate function to do this.
>
> --
> Cheers,
> Harry / Hyeonggon