[PATCH 2/2] dm verity: don't verity if readahead failed

[Wu Bo]

We found an issue under Android OTA scenario that many BIOs have to do
FEC where the data under dm-verity is 100% complete and no corruption.

Android OTA has many dm-block layers, from upper to lower:
dm-verity
dm-snapshot
dm-origin & dm-cow
dm-linear
ufs

Dm tables have to change 2 times during Android OTA merging process.
When doing table change, the dm-snapshot will be suspended for a while.
During this interval, we found there are many readahead IOs are
submitted to dm_verity from filesystem. Then the kverity works are busy
doing FEC process which cost too much time to finish dm-verity IO. And
cause system stuck.

We add some debug log and find that each readahead IO need around 10s to
finish when this situation occurred. Because here has a IO
amplification:

dm-snapshot suspend
erofs_readahead     // 300+ io is submitted
	dm_submit_bio (dm_verity)
		dm_submit_bio (dm_snapshot)
		bio return EIO
		bio got nothing, it's empty
	verity_end_io
	verity_verify_io
	forloop range(0, io->n_blocks)    // each io->nblocks ~= 20
		verity_fec_decode
		fec_decode_rsb
		fec_read_bufs
		forloop range(0, v->fec->rsn) // v->fec->rsn = 253
			new_read
			submit_bio (dm_snapshot)
		end loop
	end loop
dm-snapshot resume

Readahead BIO got nothing during dm-snapshot suspended. So all of them
will do FEC.
Each readahead BIO need to do io->n_blocks ~= 20 times verify.
Each block need to do fec, and every block need to do v->fec->rsn = 253
times read.
So during the suspend interval(~200ms), 300 readahead BIO make
300*20*253 IOs on dm-snapshot.

As readahead IO is not required by user space, and to fix this issue,
I think it would be better to pass it to upper layer to handle it.

Signed-off-by: Wu Bo <bo.wu@xxxxxxxx>
---
 drivers/md/dm-verity-target.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index 42b2483eb08c..d242e50ec869 100644
--- a/drivers/md/dm-verity-target.c
+++ b/drivers/md/dm-verity-target.c
@@ -668,7 +668,9 @@ static void verity_end_io(struct bio *bio)
 
 	verity_fec_init_io(io);
 	if (bio->bi_status &&
-	    (!verity_fec_is_enabled(io->v) || verity_is_system_shutting_down())) {
+	    (!verity_fec_is_enabled(io->v) ||
+	     verity_is_system_shutting_down() ||
+	     (bio->bi_opf & REQ_RAHEAD))) {
 		verity_finish_io(io, bio->bi_status);
 		return;
 	}
-- 
2.25.1