[PATCH] ext4: complete dropbehind writeback in task context

[Tal Zussman]

ext4_end_bio() ends writeback directly from bio completion, which
usually runs in interrupt context, unless the io_end has unwritten
extents to convert and is completed from ext4's workqueue.
folio_end_dropbehind() only invalidates the folios of an RWF_DONTCACHE
write when it runs in task context, and silently skips them otherwise.
So on ext4, dontcache writes that overwrite blocks that are already
written, or that allocate blocks without unwritten extents, leave their
folios in the page cache. Dropping them was best effort so far, as there
was no way to get task context from bio completion.

Use the task-context bio completion infrastructure instead: set
BIO_COMPLETE_IN_TASK on bios that carry dropbehind folios in
io_submit_add_bh(), so that bio_endio() runs their completion from a
worker. ext4's iomap buffered I/O path already gets this from iomap, so
only the buffer_head based writeback is affected. Skip the flag when the
io_end already has unwritten extents, as ext4 then defers the completion
itself. A later extent can still make the io_end unwritten after an
earlier bio was built. That bio's completion is then deferred twice,
which is not optimal, but harmless.

This changes behavior only for bios that carry dropbehind folios: their
completion now runs from a worker instead of in interrupt context.

On the unpatched kernel a dontcache overwrite of already-written blocks,
or a dontcache write into a new file on a dioread_lock mount, leaves
every folio in the page cache (16384/16384 for a 64M file). With the
patch, none remain in the page cache. A dontcache write into a new file
with unwritten extents already drops its folios on both kernels, as
expected, and a plain write keeps them.

Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
---
 fs/ext4/page-io.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
index 734d856a6d47..e556598ce9e1 100644
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -458,6 +458,16 @@ static void io_submit_add_bh(struct ext4_io_submit *io,
 		io_submit_init_bio(io, inode, folio, bh);
 	if (!bio_add_folio(io->io_bio, folio, bh->b_size, bh_offset(bh)))
 		goto submit_and_retry;
+
+	/*
+	 * Dropbehind folios can only be invalidated in task context. An
+	 * io_end with unwritten extents is completed from ext4's workqueue
+	 * anyway, so only ask the block layer to defer the others.
+	 */
+	if (folio_test_dropbehind(folio) &&
+	    !(io->io_end->flag & EXT4_IO_END_UNWRITTEN))
+		bio_set_flag(io->io_bio, BIO_COMPLETE_IN_TASK);
+
 	wbc_account_cgroup_owner(io->io_wbc, folio, bh->b_size);
 	io->io_next_block++;
 }

---
base-commit: 35aefefd7aba0801bab635b11943f932626be3b2
change-id: 20260924-ext4-dontcache-complete-in-task-2340ed5f3114

Best regards,
--  
Tal Zussman <tz2294@xxxxxxxxxxxx>