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

[Vlastimil Babka (SUSE)]

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