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

[Jiaqi Yan]

On Mon, Jun 15, 2026 at 11:54 PM Miaohe Lin <linmiaohe@xxxxxxxxxx> 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?

Ah, yes, thanks for pointing out, Miaohe!

>
> Thanks.
> .