Re: [PATCH v5 1/4] mm/page_alloc: only free healthy pages in high-order has_hwpoisoned folio

[Jiaqi Yan]

On Fri, Jun 12, 2026 at 11:34 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
>
> On 8 Jun 2026, at 23:44, Miaohe Lin wrote:
>
> > On 2026/5/31 13:58, 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.
> >>
> >> Another similar situation is when a transparent huge page (THP)
> >> runs into memory failure but splitting failed. Such THP will
> >> eventually be released to buddy allocator when owning userspace
> >> processes are gone, but with certain subpages having HWPoison.
> >>
> >> One obvious way to avoid both problems 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.
> >>
> >> free_has_hwpoisoned() is added in free_pages_prepare() as
> >> a shortcut and is only invoked if PG_has_hwpoisoned indicates
> >> HWPoison page exists and after checks and preparations in
> >> free_pages_prepare() all succeeded. free_has_hwpoisoned() then
> >> can re-use free_prepared_contig_range() [4] to decompose healthy
> >> ranges into the largest possible chunks of different orders.
> >> Every chunk 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 8 HWPoison
> >> pages, free_has_hwpoisoned() takes around 1ms on average on
> >> a system having 56 Intel Skylake physical cores. This is
> >> 15x to the case of freeing no HWPoison page. The cost is far
> >> from triggering soft lockup, and fair for handling exceptional
> >> hardware memory errors.
> >>
> >> [1] https://lore.kernel.org/linux-mm/1460711275-1130-15-git-send-email-mgorman@xxxxxxxxxxxxxxxxxxx
> >> [2] https://lore.kernel.org/linux-mm/1460711275-1130-16-git-send-email-mgorman@xxxxxxxxxxxxxxxxxxx
> >> [3] https://lore.kernel.org/all/20230216095131.17336-1-vbabka@xxxxxxx
> >> [4] https://lore.kernel.org/all/20260401101634.2868165-2-usama.anjum@xxxxxxx
> >>
> >> Signed-off-by: Jiaqi Yan <jiaqiyan@xxxxxxxxxx>
> >
> > Thanks for your update. This patch looks good to me while some comments below.
> >
> >> ---
> >>  mm/page_alloc.c | 85 +++++++++++++++++++++++++++++++++++++++++++++++++
> >>  1 file changed, 85 insertions(+)
> >>
> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >> index e47679e7a9db..03df929abca6 100644
> >> --- a/mm/page_alloc.c
> >> +++ b/mm/page_alloc.c
> >> @@ -208,6 +208,7 @@ gfp_t gfp_allowed_mask __read_mostly = GFP_BOOT_MASK;
> >>  unsigned int pageblock_order __read_mostly;
> >>  #endif
> >>
> >> +static void free_has_hwpoisoned(struct page *page, unsigned int order);
> >>  static void __free_pages_ok(struct page *page, unsigned int order,
> >>                          fpi_t fpi_flags);
> >>  static void reserve_highatomic_pageblock(struct page *page, int order,
> >> @@ -1309,6 +1310,14 @@ static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr)
> >>
> >>  #endif /* CONFIG_MEM_ALLOC_PROFILING */
> >>
> >> +/*
> >> + * Returns
> >> + * - true: checks and preparations all good, caller can proceed freeing.
> >> + * - false: do not proceed freeing for one of the following reasons:
> >> + *   1. Some check failed so it is not safe to proceed freeing.
> >> + *   2. A compound page has some HWPoison pages. The healthy pages
> >> + *      are already safely freed, and the HWPoison ones isolated.
> >> + */
> >>  static __always_inline bool __free_pages_prepare(struct page *page,
> >>              unsigned int order, fpi_t fpi_flags)
> >>  {
> >> @@ -1317,6 +1326,15 @@ static __always_inline bool __free_pages_prepare(struct page *page,
> >>      bool init = want_init_on_free();
> >>      bool compound = PageCompound(page);
> >>      struct folio *folio = page_folio(page);
> >> +    /*
> >> +     * When dealing with compound page, PG_has_hwpoisoned is cleared
> >> +     * with PAGE_FLAGS_SECOND. So the check must be done first.
> >> +     *
> >> +     * Note we can't exclude PG_has_hwpoisoned from PAGE_FLAGS_SECOND.
> >> +     * Because PG_has_hwpoisoned == PG_active, free_page_is_bad() will
> >> +     * confuse and complaint that the first tail page is still active.
> >> +     */
> >> +    bool should_fhh = compound && folio_test_has_hwpoisoned(folio);
> >>
> >>      if (fpi_flags & FPI_PREPARED)
> >>              return true;
> >> @@ -1443,6 +1461,16 @@ static __always_inline bool __free_pages_prepare(struct page *page,
> >>
> >>      debug_pagealloc_unmap_pages(page, 1 << order);
> >>
> >> +    /*
> >> +     * After breaking down compound page and dealing with page metadata
> >> +     * (e.g. page owner and page alloc tags), take a shortcut if this
> >> +     * was a compound page containing certain HWPoison subpages.
> >> +     */
> >> +    if (should_fhh) {
> >> +            free_has_hwpoisoned(page, order);
> >> +            return false;
> >> +    }
> >
> > When the code reaches here, the hwpoisoned pages have passed through kernel_poison_pages,
> > kasan_poison_pages, kernel_init_pages, arch_free_page... These functions might write to
> > the hwpoisoned pages. Is it safe to do so?
>
> At least, kernel_poison_pages() writes to the page. It probably should be
> moved up, somewhere like above kernel_poison_pages().

Writing to HWPoison pages (location having memory error) is usually
safe, as in it doesn't cause a machine check exception. Memory
controller usually just fails the write op and waits for the next read
to raise the MCE / exception for prevent silent data corruption.

>
> I do not like the shortcut method, since the pages are freed in
> __free_pages_prepare(). This causes confusion. One alternative I can think

What exactly is the confusion? or why does freeing has to be done by
__free_pages_prepare()'s caller?

Harry suggested the shortcut method [1]. Although freeing inline might
surprise the caller, it simplifies things for all callers.

[1] https://lore.kernel.org/linux-mm/aVy7L-3pc4JUYBEn@hyeyoo

> of is to make __free_pages_prepare() returns a enum
> { FREE_PAGE_PREPARE_SUCCESS, FREE_PAGE_PREPARE_FAIL, FREE_PAGE_PREPARE_HAS_HWPOISON}
> and handle the return value in the caller.

Are you worried that a caller of __free_pages_prepare() may see false
returned in the has_hwpoison case, but mistakenly propagate some kind
of error, or doing something under the assumption that folio not
freed? In this case the three enums can be useful. But I checked
current callers of __free_pages_prepare() and they don't have the
above problem.

>
>
> Best Regards,
> Yan, Zi