[PATCH vfs.tmpfs 5/5] mm: invalidation check mapping before folio_contains

[Hugh Dickins <hughd@google.com>]

Enabling tmpfs "direct IO" exposes it to invalidate_inode_pages2_range(),
which when swapping can hit the VM_BUG_ON_FOLIO(!folio_contains()): the
folio has been moved from page cache to swap cache (with folio->mapping
reset to NULL), but the folio_index() embedded in folio_contains() sees
swapcache, and so returns the swapcache_index() - whereas folio->index
would be the right one to check against the index from mapping's xarray.

There are different ways to fix this, but my preference is just to order
the checks in invalidate_inode_pages2_range() the same way that they are
in __filemap_get_folio() and find_lock_entries() and filemap_fault():
check folio->mapping before folio_contains().

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/truncate.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/truncate.c b/mm/truncate.c
index 95d1291d269b..c3320e66d6ea 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -657,11 +657,11 @@ int invalidate_inode_pages2_range(struct address_space *mapping,
 			}
 
 			folio_lock(folio);
-			VM_BUG_ON_FOLIO(!folio_contains(folio, indices[i]), folio);
-			if (folio->mapping != mapping) {
+			if (unlikely(folio->mapping != mapping)) {
 				folio_unlock(folio);
 				continue;
 			}
+			VM_BUG_ON_FOLIO(!folio_contains(folio, indices[i]), folio);
 			folio_wait_writeback(folio);
 
 			if (folio_mapped(folio))
-- 
2.35.3