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

[Miaohe Lin]

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.
.