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

[Jiaqi Yan]

On Mon, Jun 22, 2026 at 1:30 AM Vlastimil Babka (SUSE)
<vbabka@xxxxxxxxxx> wrote:
>
> On 6/22/26 03:12, Jiaqi Yan wrote:
> > 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?
>
> Sure, let's see how it will look like.
>
> > 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.
>
> If it helps, why not. Maybe have a new fpi_t flag instead of a new bool then?

Yeah, thanks for the good idea! It is done in v6 that I just sent out.

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