ext4: Fix stale xarray tags after writeback

There are cases where ext4_bio_write_page() gets called for a page which has no buffers to submit. This happens e.g. when the part of the file is actually a hole, when we cannot allocate blocks due to being called from jbd2, or in data=journal mode when checkpointing writes the buffers earlier. In these cases we just return from ext4_bio_write_page() however if the page didn't need redirtying, we will leave stale DIRTY and/or TOWRITE tags in xarray because those get cleared only in __folio_start_writeback(). As a result we can leave these tags set in mappings even after a final sync on filesystem that's getting remounted read-only or that's being frozen. Various assertions can then get upset when writeback is started on such filesystems (Gerald reported assertion in ext4_journal_check_start() firing).

Fix the problem by cycling the page through writeback state even if we decide nothing needs to be written for it so that xarray tags get properly updated. This is slightly silly (we could update the xarray tags directly) but I don't think a special helper messing with xarray tags is really worth it in this relatively rare corner case.

---
 fs/ext4/page-io.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

index 39abfeec5f36..0a3ef9bd6803 100644
@@ -523,9 +523,15 @@ int ext4_bio_write_folio(struct ext4_io_submit *io, struct folio *folio,
 		nr_to_submit++;
 	} while ((bh = bh->b_this_page) != head);
 
-	/* Nothing to submit? Just unlock the folio... */
-	if (!nr_to_submit)
+	if (!nr_to_submit) {
+		/*
+		 * We have nothing to submit. Just cycle the folio through
+		 * writeback state to properly update xarray tags.
+		 */
+		__folio_start_writeback(folio, keep_towrite);
+		folio_end_writeback(folio);
 		return 0;
+	}
 
 	bh = head = folio_buffers(folio);
 

2.51.0

Applied, thanks!

[1/1] ext4: Fix stale xarray tags after writeback

      commit: bc6874ab41809f210c3e966576374d0778e144ff

Best regards,

Theodore Ts'o <[email protected]>