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

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 6/22/26 03:12, Jiaqi Yan wrote:
> On Thu, Jun 18, 2026 at 8:02 AM Vlastimil Babka (SUSE)
> <vbabka@kernel.org> 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@nvidia.com> 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@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
>> >>>>> [4] https://lore.kernel.org/all/20260401101634.2868165-2-usama.anjum@arm.com
>> >>>>>
>> >>>>> Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
>> >>>>
>> >>>> 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?

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