[PATCH 0/2] dm verity: fix FEC stuck during lower dm suspend

[PATCH 0/2] dm verity: fix FEC stuck during lower dm suspend

[Wu Bo]

Hi,

We found an issue under Android OTA scenario that many readahead BIOs have
to do FEC and caused system stuck. These 2 patches try to fix this issue.

Thanks

Wu Bo (2):
  dm verity: init fec io before cleaning it
  dm verity: don't verity if readahead failed

 drivers/md/dm-verity-target.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

-- 
2.25.1

[PATCH 1/2] dm verity: init fec io before cleaning it

[Wu Bo]

If BIO error, it may goto verity_finish_io() before
verity_fec_init_io(). Therefor, the fec_io->rs is not initialized and
may crash when doing memory freeing in verity_fec_finish_io().

Crash call stack:
 die+0x90/0x2b8
 __do_kernel_fault+0x260/0x298
 do_bad_area+0x2c/0xdc
 do_translation_fault+0x3c/0x54
 do_mem_abort+0x54/0x118
 el1_abort+0x38/0x5c
 el1h_64_sync_handler+0x50/0x90
 el1h_64_sync+0x64/0x6c
 free_rs+0x18/0xac
 fec_rs_free+0x10/0x24
 mempool_free+0x58/0x148
 verity_fec_finish_io+0x4c/0xb0
 verity_end_io+0xb8/0x150

Signed-off-by: Wu Bo <bo.wu@vivo.com>
---
 drivers/md/dm-verity-target.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index e115fcfe723c..42b2483eb08c 100644
--- a/drivers/md/dm-verity-target.c
+++ b/drivers/md/dm-verity-target.c
@@ -642,7 +642,6 @@ static void verity_work(struct work_struct *w)
 
 	io->in_tasklet = false;
 
-	verity_fec_init_io(io);
 	verity_finish_io(io, errno_to_blk_status(verity_verify_io(io)));
 }
 
@@ -667,6 +666,7 @@ static void verity_end_io(struct bio *bio)
 {
 	struct dm_verity_io *io = bio->bi_private;
 
+	verity_fec_init_io(io);
 	if (bio->bi_status &&
 	    (!verity_fec_is_enabled(io->v) || verity_is_system_shutting_down())) {
 		verity_finish_io(io, bio->bi_status);
-- 
2.25.1

Re: [PATCH 1/2] dm verity: init fec io before cleaning it

[Eric Biggers]

On Tue, Nov 21, 2023 at 01:55:28AM -0700, Wu Bo wrote:
> If BIO error, it may goto verity_finish_io() before
> verity_fec_init_io(). Therefor, the fec_io->rs is not initialized and
> may crash when doing memory freeing in verity_fec_finish_io().
> 
> Crash call stack:
>  die+0x90/0x2b8
>  __do_kernel_fault+0x260/0x298
>  do_bad_area+0x2c/0xdc
>  do_translation_fault+0x3c/0x54
>  do_mem_abort+0x54/0x118
>  el1_abort+0x38/0x5c
>  el1h_64_sync_handler+0x50/0x90
>  el1h_64_sync+0x64/0x6c
>  free_rs+0x18/0xac
>  fec_rs_free+0x10/0x24
>  mempool_free+0x58/0x148
>  verity_fec_finish_io+0x4c/0xb0
>  verity_end_io+0xb8/0x150
> 
> Signed-off-by: Wu Bo <bo.wu@vivo.com>

What commit introduced this bug?  It looks like it was commit 5721d4e5a9cd ("dm
verity: Add optional "try_verify_in_tasklet" feature"), which moved the call to
verity_fec_init_io() from verity_map() to verity_work().  Do you agree?  Should
the call to verity_fec_init_io() perhaps be moved back to verity_map(), instead
of putting it in verity_end_io()?  Or maybe another call to verity_fec_init_io()
should be added only just before the call to verity_finish_io() in
verity_end_io(), given that it's not needed in the tasklet case?

In any case, please also add a Fixes tag with the appropriate commit, as well as
a "Cc stable" tag.

- Eric

[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@vivo.com>
---
 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

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

[Eric Biggers]

On Tue, Nov 21, 2023 at 01:55:29AM -0700, Wu Bo wrote:
> 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@vivo.com>
> ---
>  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;
>  	}

Thanks, this seems reasonable to me.  As with your previous patch: what commit
introduced this issue?  To me this looks like a longstanding issue, maybe dating
back to the original addition of FEC support to dm-verity by commit a739ff3f543a
("dm verity: add support for forward error correction"); do you agree?  Can you
please add Fixes and "Cc stable" tags to your patch?  Thanks!

- Eric