Re: [PATCH] mm/page_alloc: fix initialization of tags of the huge zero folio with init_on_free

David Hildenbrand (Arm)
2026-04-21T08:07:02Z

On 4/21/26 09:06, Dev Jain wrote:
>
>
> On 21/04/26 2:46 am, David Hildenbrand (Arm) wrote:
>> __GFP_ZEROTAGS semantics are currently a bit weird, but effectively this
>> flag is only ever set alongside __GFP_ZERO and __GFP_SKIP_KASAN.
>>
>> If we run with init_on_free, we will zero out pages during
>> __free_pages_prepare(), to skip zeroing on the allocation path.
>>
>> However, when allocating with __GFP_ZEROTAG set, post_alloc_hook() will
>> consequently not only skip clearing page content, but also skip
>> clearing tag memory.
>>
>> Not clearing tags through __GFP_ZEROTAGS is irrelevant for most pages that
>> will get mapped to user space through set_pte_at() later: set_pte_at() and
>> friends will detect that the tags have not been initialized yet
>> (PG_mte_tagged not set), and initialize them.
>>
>> However, for the huge zero folio, which will be mapped through a PMD
>> marked as special, this initialization will not be performed, ending up
>> exposing whatever tags were still set for the pages.
>>
>> The docs (Documentation/arch/arm64/memory-tagging-extension.rst) state
>> that allocation tags are set to 0 when a page is first mapped to user
>> space. That no longer holds with the huge zero folio when init_on_free
>> is enabled.
>>
>> Fix it by decoupling __GFP_ZEROTAGS from __GFP_ZERO, passing to
>> tag_clear_highpages() whether we want to also clear page content.
>>
>> As we are touching the interface either way, just clean it up by
>> only calling it when HW tags are enabled, dropping the return value, and
>> dropping the common code stub.
>>
>> Reproduced with the huge zero folio by modifying the check_buffer_fill
>> arm64/mte selftest to use a 2 MiB area, after making sure that pages have
>> a non-0 tag set when freeing (note that, during boot, we will not
>> actually initialize tags, but only set KASAN_TAG_KERNEL in the page
>> flags).
>>
>> $ ./check_buffer_fill
>> 1..20
>> ...
>> not ok 17 Check initial tags with private mapping, sync error mode and mmap memory
>> not ok 18 Check initial tags with private mapping, sync error mode and mmap/mprotect memory
>> ...
>>
>> This code needs more cleanups; we'll tackle that next, like
>> decoupling __GFP_ZEROTAGS from __GFP_SKIP_KASAN, moving all the
>> KASAN magic into a separate helper, and consolidating HW-tag handling.
>>
>> Fixes: adfb6609c680 ("mm/huge_memory: initialise the tags of the huge zero folio")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>> ---
>> arch/arm64/include/asm/page.h | 3 ---
>> arch/arm64/mm/fault.c | 16 +++++-----------
>> include/linux/gfp_types.h | 10 +++++-----
>> include/linux/highmem.h | 10 +---------
>> mm/page_alloc.c | 12 +++++++-----
>> 5 files changed, 18 insertions(+), 33 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
>> index e25d0d18f6d7..5c6cbfbbd34c 100644
>> --- a/arch/arm64/include/asm/page.h
>> +++ b/arch/arm64/include/asm/page.h
>> @@ -33,9 +33,6 @@ struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
>> unsigned long vaddr);
>> #define vma_alloc_zeroed_movable_folio vma_alloc_zeroed_movable_folio
>>
>> -bool tag_clear_highpages(struct page *to, int numpages);
>> -#define __HAVE_ARCH_TAG_CLEAR_HIGHPAGES
>> -
>> #define copy_user_page(to, from, vaddr, pg) copy_page(to, from)
>>
>> typedef struct page *pgtable_t;
>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>> index 0f3c5c7ca054..32a3723f2d34 100644
>> --- a/arch/arm64/mm/fault.c
>> +++ b/arch/arm64/mm/fault.c
>> @@ -1018,21 +1018,15 @@ struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
>> return vma_alloc_folio(flags, 0, vma, vaddr);
>> }
>>
>> -bool tag_clear_highpages(struct page *page, int numpages)
>> +void tag_clear_highpages(struct page *page, int numpages, bool clear_pages)
>> {
>> - /*
>> - * Check if MTE is supported and fall back to clear_highpage().
>> - * get_huge_zero_folio() unconditionally passes __GFP_ZEROTAGS and
>> - * post_alloc_hook() will invoke tag_clear_highpages().
>> - */
>> - if (!system_supports_mte())
>> - return false;
>> -
>> /* Newly allocated pages, shouldn't have been tagged yet */
>> for (int i = 0; i < numpages; i++, page++) {
>> WARN_ON_ONCE(!try_page_mte_tagging(page));
>> - mte_zero_clear_page_tags(page_address(page));
>> + if (clear_pages)
>> + mte_zero_clear_page_tags(page_address(page));
>> + else
>> + mte_clear_page_tags(page_address(page));
>> set_page_mte_tagged(page);
>> }
>> - return true;
>> }
>> diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
>> index 6c75df30a281..fd53a6fba33f 100644
>> --- a/include/linux/gfp_types.h
>> +++ b/include/linux/gfp_types.h
>> @@ -273,11 +273,11 @@ enum {
>> *
>> * %__GFP_ZERO returns a zeroed page on success.
>> *
>> - * %__GFP_ZEROTAGS zeroes memory tags at allocation time if the memory itself
>> - * is being zeroed (either via __GFP_ZERO or via init_on_alloc, provided that
>> - * __GFP_SKIP_ZERO is not set). This flag is intended for optimization: setting
>> - * memory tags at the same time as zeroing memory has minimal additional
>> - * performance impact.
>> + * %__GFP_ZEROTAGS zeroes memory tags at allocation time. This flag is intended
>> + * for optimization: setting memory tags at the same time as zeroing memory
>> + * (e.g., with __GPF_ZERO) has minimal additional performance impact. However,
>> + * __GFP_ZEROTAGS also zeroes the tags even if memory is not getting zeroed at
>> + * allocation time (e.g., with init_on_free).
>> *
>> * %__GFP_SKIP_KASAN makes KASAN skip unpoisoning on page allocation.
>> * Used for userspace and vmalloc pages; the latter are unpoisoned by
>> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
>> index af03db851a1d..62f589baa343 100644
>> --- a/include/linux/highmem.h
>> +++ b/include/linux/highmem.h
>> @@ -345,15 +345,7 @@ static inline void clear_highpage_kasan_tagged(struct page *page)
>> kunmap_local(kaddr);
>> }
>>
>> -#ifndef __HAVE_ARCH_TAG_CLEAR_HIGHPAGES
>> -
>> -/* Return false to let people know we did not initialize the pages */
>> -static inline bool tag_clear_highpages(struct page *page, int numpages)
>> -{
>> - return false;
>> -}
>> -
>> -#endif
>> +void tag_clear_highpages(struct page *to, int numpages, bool clear_pages);
>>
>> /*
>> * If we pass in a base or tail page, we can zero up to PAGE_SIZE.
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 65e205111553..8c6821d25a00 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1808,9 +1808,9 @@ static inline bool should_skip_init(gfp_t flags)
>> inline void post_alloc_hook(struct page *page, unsigned int order,
>> gfp_t gfp_flags)
>> {
>> + const bool zero_tags = kasan_hw_tags_enabled() && (gfp_flags & __GFP_ZEROTAGS);
>
> Sashiko:
>
> https://sashiko.dev/#/patchset/20260420-zerotags-v1-1-3edc93e95bb4%40kernel.org
>
> PROT_MTE works without KASAN_HW_TAGS, so probably just retain the
> system_supports_mte() check in tag_clear_highpages(), and document
> that GFP_ZEROTAGS is only for MTE?

Right, we have to clear tags here even without kasan. God, what an ugly
mess people created here with these GFP flags.

Let me think about how to do that in the least ugly way.

--
Cheers,

David