[PATCH 5.15.y] mm/huge_memory: fix truncate racing with PMD zapping

[Sasha Levin <sashal@kernel.org>]

From: Chris Mason <mason@kernel.org>

[ Upstream commit e6a07d29b0a60f3a4cfe77fe34eeb4b3fb38d6c4 ]

zap_huge_pmd_folio() removes the rmap before calling folio_mark_dirty().
If this was the last mapping, truncation can skip unmapping the folio
and proceed without waiting for the PMD lock, racing with dirtying.

ifs_set_range_dirty() can oops if MADV_DONTNEED races with
ftruncate() on XFS, when we get the folio set dirty after it was found
to not be mapped anymore.

Move folio_mark_dirty() and folio_mark_accessed() before rmap removal,
matching the PTE zap path. This keeps the folio mapped while updating
its state, so truncation must wait for the page table lock.

Fix generated by an LLM and sent by Chris Mason in private.

Fixes: fe6cf984939d ("mm/huge_memory: transfer the pmd dirty bit to the folio on zap")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
[ david: simplify comment and description ]
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-13-d6437171ecc3@kernel.org
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
[ sashal: adapted zap_huge_pmd_folio() changes to the page-based implementation in zap_huge_pmd() ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/huge_memory.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index cd44744724d0a..1cfc6c8156e18 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1622,8 +1622,6 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 
 		if (pmd_present(orig_pmd)) {
 			page = pmd_page(orig_pmd);
-			page_remove_rmap(page, true);
-			VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
 			VM_BUG_ON_PAGE(!PageHead(page), page);
 		} else if (thp_migration_supported()) {
 			swp_entry_t entry;
@@ -1647,6 +1645,12 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 				set_page_dirty(page);
 		}
 
+		/* Remove the rmap after transferring the dirty bit to the page. */
+		if (pmd_present(orig_pmd)) {
+			page_remove_rmap(page, true);
+			VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
+		}
+
 		spin_unlock(ptl);
 		if (flush_needed)
 			tlb_remove_page_size(tlb, page, HPAGE_PMD_SIZE);
-- 
2.53.0