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.

Fixes: 336bf30eb765 (hugetlbfs: fix anon huge page migration race) Reported-by: syzbot+2d9c96466c978346b55f@syzkaller.appspotmail.com Link: https://lore.kernel.org/all/68e9715a.050a0220.1186a4.000d.GAE@google.com Debugged-by: Lance Yang lance.yang@linux.dev Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org Cc: stable@vger.kernel.org --- mm/migrate.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c index 5169f9717f60..4688b9e38cd2 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -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);