mm/huge_memory: initialise the tags of the huge zero folio

On arm64 with MTE enabled, a page mapped as Normal Tagged (PROT_MTE) in
user space will need to have its allocation tags initialised.  This is
normally done in the arm64 set_pte_at() after checking the memory
attributes.  Such page is also marked with the PG_mte_tagged flag to avoid
subsequent clearing.  Since this relies on having a struct page,
pte_special() mappings are ignored.

Commit d82d09e ("mm/huge_memory: mark PMD mappings of the huge zero
folio special") maps the huge zero folio special and the arm64
set_pmd_at() will no longer zero the tags.  There is no guarantee that the
tags are zero, especially if parts of this huge page have been previously
tagged.

It's fairly easy to detect this by regularly dropping the caches to
force the reallocation of the huge zero folio.

Allocate the huge zero folio with the __GFP_ZEROTAGS flag.  In addition,
do not warn in the arm64 __access_remote_tags() when reading tags from the
huge zero page.

I bundled the arm64 change in here as well since they are both related to
the commit mapping the huge zero folio as special.

[catalin.marinas@arm.com: handle arch mte_zero_clear_page_tags() code issuing MTE instructions]
  Link: https://lkml.kernel.org/r/aQi8dA_QpXM8XqrE@arm.com
Link: https://lkml.kernel.org/r/20251031170133.280742-1-catalin.marinas@arm.com
Fixes: d82d09e ("mm/huge_memory: mark PMD mappings of the huge zero folio special")
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Tested-by: Beleswar Padhi <b-padhi@ti.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Aishwarya TCV <aishwarya.tcv@arm.com>
Cc: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ctmarinas

arch/arm64/kernel/mte.c

@@ -476,7 +476,8 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
 
 		folio = page_folio(page);
 		if (folio_test_hugetlb(folio))
-			WARN_ON_ONCE(!folio_test_hugetlb_mte_tagged(folio));
+			WARN_ON_ONCE(!folio_test_hugetlb_mte_tagged(folio) &&
+				     !is_huge_zero_folio(folio));
 		else
 			WARN_ON_ONCE(!page_mte_tagged(page) && !is_zero_page(page));
 

arch/arm64/mm/fault.c

@@ -969,6 +969,16 @@ struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
 
 void tag_clear_highpage(struct page *page)
 {
+	/*
+	 * 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_highpage().
+	 */
+	if (!system_supports_mte()) {
+		clear_highpage(page);
+		return;
+	}
+
 	/* Newly allocated page, shouldn't have been tagged yet */
 	WARN_ON_ONCE(!try_page_mte_tagging(page));
 	mte_zero_clear_page_tags(page_address(page));

mm/huge_memory.c

@@ -214,7 +214,8 @@ static bool get_huge_zero_folio(void)
 	if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
 		return true;
 
-	zero_folio = folio_alloc((GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE,
+	zero_folio = folio_alloc((GFP_TRANSHUGE | __GFP_ZERO | __GFP_ZEROTAGS) &
+				 ~__GFP_MOVABLE,
 			HPAGE_PMD_ORDER);
 	if (!zero_folio) {
 		count_vm_event(THP_ZERO_PAGE_ALLOC_FAILED);