FAILED: patch "[PATCH] migrate: correct lock ordering for hugetlb file folios" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x b7880cb166ab62c2409046b2347261abf701530e
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026012708-eardrum-operation-6193@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From b7880cb166ab62c2409046b2347261abf701530e Mon Sep 17 00:00:00 2001
From: "Matthew Wilcox (Oracle)" <[email protected]>
Date: Fri, 9 Jan 2026 04:13:42 +0000
Subject: [PATCH] migrate: correct lock ordering for hugetlb file folios

Syzbot has found a deadlock (analyzed by Lance Yang):

1) Task (5749): Holds folio_lock, then tries to acquire i_mmap_rwsem(read lock).
2) Task (5754): Holds i_mmap_rwsem(write lock), then tries to acquire folio_lock.

migrate_pages()
-> migrate_hugetlbs()

    -> unmap_and_move_huge_page()     <- Takes folio_lock!
      -> remove_migration_ptes()
        -> __rmap_walk_file()
          -> i_mmap_lock_read()       <- Waits for i_mmap_rwsem(read lock)!

hugetlbfs_fallocate()
-> hugetlbfs_punch_hole() <- Takes i_mmap_rwsem(write lock)!

    -> hugetlbfs_zero_partial_page()
     -> filemap_lock_hugetlb_folio()
      -> filemap_lock_folio()
        -> __filemap_get_folio        <- Waits for folio_lock!

The migration path is the one taking locks in the wrong order according to the documentation at the top of mm/rmap.c. So expand the scope of the existing i_mmap_lock to cover the calls to remove_migration_ptes() too.

This is (mostly) how it used to be after commit c0d0381ade79. That was removed by 336bf30eb765 for both file & anon hugetlb pages when it should only have been removed for anon hugetlb pages.

index 5169f9717f60..4688b9e38cd2 100644
@@ -1458,6 +1458,7 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio,
 	int page_was_mapped = 0;
 	struct anon_vma *anon_vma = NULL;
 	struct address_space *mapping = NULL;
+	enum ttu_flags ttu = 0;
 
 	if (folio_ref_count(src) == 1) {
 		/* page was freed from under us. So we are done. */

@@ -1498,8 +1499,6 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio,
 		goto put_anon;
 
 	if (folio_mapped(src)) {
-		enum ttu_flags ttu = 0;
-
 		if (!folio_test_anon(src)) {
 			/*
 			 * In shared mappings, try_to_unmap could potentially

@@ -1516,16 +1515,17 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio,
 
 		try_to_migrate(src, ttu);
 		page_was_mapped = 1;
-
-		if (ttu & TTU_RMAP_LOCKED)
-			i_mmap_unlock_write(mapping);
 	}
 
 	if (!folio_mapped(src))
 		rc = move_to_new_folio(dst, src, mode);
 
 	if (page_was_mapped)
-		remove_migration_ptes(src, !rc ? dst : src, 0);
+		remove_migration_ptes(src, !rc ? dst : src,
+				ttu ? RMP_LOCKED : 0);
+
+	if (ttu & TTU_RMAP_LOCKED)
+		i_mmap_unlock_write(mapping);
 
 unlock_put_anon:
 	folio_unlock(dst);

Syzbot has found a deadlock (analyzed by Lance Yang):

1) Task (5749): Holds folio_lock, then tries to acquire i_mmap_rwsem(read lock).
2) Task (5754): Holds i_mmap_rwsem(write lock), then tries to acquire folio_lock.

migrate_pages()
-> migrate_hugetlbs()

    -> unmap_and_move_huge_page()     <- Takes folio_lock!
      -> remove_migration_ptes()
        -> __rmap_walk_file()
          -> i_mmap_lock_read()       <- Waits for i_mmap_rwsem(read lock)!

hugetlbfs_fallocate()
-> hugetlbfs_punch_hole() <- Takes i_mmap_rwsem(write lock)!

    -> hugetlbfs_zero_partial_page()
     -> filemap_lock_hugetlb_folio()
      -> filemap_lock_folio()
        -> __filemap_get_folio        <- Waits for folio_lock!

The migration path is the one taking locks in the wrong order according to the documentation at the top of mm/rmap.c. So expand the scope of the existing i_mmap_lock to cover the calls to remove_migration_ptes() too.

This is (mostly) how it used to be after commit c0d0381ade79. That was removed by 336bf30eb765 for both file & anon hugetlb pages when it should only have been removed for anon hugetlb pages.

---
 mm/migrate.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

index 3050dd85910a..728fbb0b9024 100644
@@ -1291,6 +1291,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 	struct page *new_hpage;
 	struct anon_vma *anon_vma = NULL;
 	struct address_space *mapping = NULL;
+	enum ttu_flags ttu = 0;
 
 	/*
 	 * Migratability of hugepages depends on architectures and their size.

@@ -1344,9 +1345,6 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 		goto put_anon;
 
 	if (page_mapped(hpage)) {
-		bool mapping_locked = false;
-		enum ttu_flags ttu = 0;
-
 		if (!PageAnon(hpage)) {
 			/*
 			 * In shared mappings, try_to_unmap could potentially

@@ -1358,15 +1356,11 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 			if (unlikely(!mapping))
 				goto unlock_put_anon;
 
-			mapping_locked = true;
 			ttu |= TTU_RMAP_LOCKED;
 		}
 
 		try_to_migrate(hpage, ttu);
 		page_was_mapped = 1;
-
-		if (mapping_locked)
-			i_mmap_unlock_write(mapping);
 	}
 
 	if (!page_mapped(hpage))

@@ -1374,7 +1368,11 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 
 	if (page_was_mapped)
 		remove_migration_ptes(hpage,
-			rc == MIGRATEPAGE_SUCCESS ? new_hpage : hpage, false);
+			rc == MIGRATEPAGE_SUCCESS ? new_hpage : hpage,
+				ttu ? true : false);
+
+	if (ttu & TTU_RMAP_LOCKED)
+		i_mmap_unlock_write(mapping);
 
 unlock_put_anon:
 	unlock_page(new_hpage);

2.47.3