Re: [PATCH] dm verity: don't use WQ_MEM_RECLAIM

[Mikulas Patocka <mpatocka@redhat.com>]

On Tue, 3 Sep 2024, Eric Biggers wrote:

> From: Eric Biggers <ebiggers@google.com>
> 
> Since dm-verity doesn't support writes, the kernel's memory reclaim code
> will never wait on dm-verity work.  That makes the use of WQ_MEM_RECLAIM
> in dm-verity unnecessary.  WQ_MEM_RECLAIM has been present from the
> beginning of dm-verity, but I could not find a justification for it;
> I suspect it was just copied from dm-crypt which does support writes.
> 
> Therefore, remove WQ_MEM_RECLAIM from dm-verity.  This eliminates the
> creation of an unnecessary rescuer thread per dm-verity device.
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Hmm. I can think about a case where you have read-only dm-verity device, 
on the top of that you have dm-snapshot device and on the top of that you 
have a writable filesystem.

When the filesystem needs to write data, it submits some write bios. When 
dm-snapshot receives these write bios, it will read from the dm-verity 
device and write to the snapshot's exception store device. So, dm-verity 
needs WQ_MEM_RECLAIM in this case.

Mikulas

> ---
>  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 cf659c8feb29f..051e84ca401dc 100644
> --- a/drivers/md/dm-verity-target.c
> +++ b/drivers/md/dm-verity-target.c
> @@ -1488,11 +1488,11 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
>  	 * Also as required for the "try_verify_in_tasklet" feature: WQ_HIGHPRI
>  	 * allows verify_wq to preempt softirq since verification in BH workqueue
>  	 * will fall-back to using it for error handling (or if the bufio cache
>  	 * doesn't have required hashes).
>  	 */
> -	v->verify_wq = alloc_workqueue("kverityd", WQ_MEM_RECLAIM | WQ_HIGHPRI, 0);
> +	v->verify_wq = alloc_workqueue("kverityd", WQ_HIGHPRI, 0);
>  	if (!v->verify_wq) {
>  		ti->error = "Cannot allocate workqueue";
>  		r = -ENOMEM;
>  		goto bad;
>  	}
> 
> base-commit: 88fac17500f4ea49c7bac136cf1b27e7b9980075
> -- 
> 2.46.0
>