From 6a288a4ddb4a994490505ab5f41c445f8e6b6467 Mon Sep 17 00:00:00 2001 From: "David Hildenbrand (Arm)" Date: Tue, 21 Apr 2026 17:39:07 +0200 Subject: [PATCH] mm/page_alloc: fix initialization of tags of the huge zero folio with init_on_free __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. Invert the meaning of the tag_clear_highpages() return value to have clearer semantics. 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. [akpm@linux-foundation.org: s/__GPF_ZERO/__GFP_ZERO/, per David] Link: https://lore.kernel.org/20260421-zerotags-v2-1-05cb1035482e@kernel.org Fixes: adfb6609c680 ("mm/huge_memory: initialise the tags of the huge zero folio") Signed-off-by: David Hildenbrand (Arm) Reviewed-by: Catalin Marinas Tested-by: Lance Yang Cc: Brendan Jackman Cc: Dev Jain Cc: Johannes Weiner Cc: Liam Howlett Cc: Lorenzo Stoakes (Oracle) Cc: Mark Brown Cc: Michal Hocko Cc: Mike Rapoport Cc: Ryan Roberts Cc: Suren Baghdasaryan Cc: Will Deacon Cc: Zi Yan Cc: Signed-off-by: Andrew Morton --- arch/arm64/include/asm/page.h | 2 +- arch/arm64/mm/fault.c | 11 +++++++---- include/linux/gfp_types.h | 10 +++++----- include/linux/highmem.h | 7 ++++--- mm/page_alloc.c | 8 ++++---- 5 files changed, 21 insertions(+), 17 deletions(-) diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h index e25d0d18f6d7..58200de8a221 100644 --- a/arch/arm64/include/asm/page.h +++ b/arch/arm64/include/asm/page.h @@ -33,7 +33,7 @@ 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); +bool tag_clear_highpages(struct page *to, int numpages, bool clear_pages); #define __HAVE_ARCH_TAG_CLEAR_HIGHPAGES #define copy_user_page(to, from, vaddr, pg) copy_page(to, from) diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c index 0f3c5c7ca054..739800835920 100644 --- a/arch/arm64/mm/fault.c +++ b/arch/arm64/mm/fault.c @@ -1018,7 +1018,7 @@ 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) +bool tag_clear_highpages(struct page *page, int numpages, bool clear_pages) { /* * Check if MTE is supported and fall back to clear_highpage(). @@ -1026,13 +1026,16 @@ bool tag_clear_highpages(struct page *page, int numpages) * post_alloc_hook() will invoke tag_clear_highpages(). */ if (!system_supports_mte()) - return false; + return clear_pages; /* 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; + return false; } diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h index 6c75df30a281..cd4972a7c97c 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. Setting memory tags at + * the same time as zeroing memory (e.g., with __GFP_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..d7aac9de1c8a 100644 --- a/include/linux/highmem.h +++ b/include/linux/highmem.h @@ -347,10 +347,11 @@ static inline void clear_highpage_kasan_tagged(struct page *page) #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) +/* Returns true if the caller has to initialize the pages */ +static inline bool tag_clear_highpages(struct page *page, int numpages, + bool clear_pages) { - return false; + return clear_pages; } #endif diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 227d58dc3de6..23c7298d3be2 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 = gfp_flags & __GFP_ZEROTAGS; bool init = !want_init_on_free() && want_init_on_alloc(gfp_flags) && !should_skip_init(gfp_flags); - bool zero_tags = init && (gfp_flags & __GFP_ZEROTAGS); int i; set_page_private(page, 0); @@ -1832,11 +1832,11 @@ inline void post_alloc_hook(struct page *page, unsigned int order, */ /* - * If memory tags should be zeroed - * (which happens only when memory should be initialized as well). + * Clearing tags can efficiently clear the memory for us as well, if + * required. */ if (zero_tags) - init = !tag_clear_highpages(page, 1 << order); + init = tag_clear_highpages(page, 1 << order, /* clear_pages= */init); if (!should_skip_kasan_unpoison(gfp_flags) && kasan_unpoison_pages(page, order, init)) {