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

[Zi Yan]

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().

I do not like the shortcut method, since the pages are freed in
__free_pages_prepare(). This causes confusion. One alternative I can think
of is to make __free_pages_prepare() returns a enum
{ FREE_PAGE_PREPARE_SUCCESS, FREE_PAGE_PREPARE_FAIL, FREE_PAGE_PREPARE_HAS_HWPOISON}
and handle the return value in the caller.


Best Regards,
Yan, Zi