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

[Jiaqi Yan]

On Thu, Jun 18, 2026 at 8:02 AM Vlastimil Babka (SUSE)
<vbabka@xxxxxxxxxx> wrote:
>
> On 6/16/26 08:53, Miaohe Lin wrote:
> > On 2026/6/16 11:23, Jiaqi Yan wrote:
> >> 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
> >
> > In x86, set_mce_nospec is called for hwpoisoned pages. So writing to
> > HWPoison pages would cause unexpected page fault in kernel?
> >
> > Thanks.
>
> Seems we'll have to extract everything between kernel_poison_pages() and
> debug_pagealloc_unmap_pages() to a function, don't call it when should_fhh
> is true and instead call it in free_has_hwpoisoned() on pages that are not
> HWPoison? I think it's acceptable to do it there one page at a time in order

Let's say I extract everything between kernel_poison_pages() and
debug_pagealloc_unmap_pages() into __free_pages_sanitize(page, order).
How about adding an extra bool to let __free_prepared_contig_range()
to do __free_pages_sanitize() for a block of pages?

This doesn't really matter to kernel_poison_pages() and
clear_highpages_kasan_tagged() because they deal with pages one by
one. However, it seems more efficient for kasan_poison() and
debug_pagealloc_unmap_pages().

BTW, I wonder whether I should carry over the fpi_t flags from
__free_pages_prepare() into free_has_hwpoisoned() and
__free_prepared_contig_range()? Right now the intended code paths to
get to __free_pages_prepare() all just have FPI_NONE, e.g.
free_frozen_pages() and free_unref_folios(). So I didn't complicate
things further.

> not to complicate things as most of that stuff is debug-only and we're
> handling a rare event.