Re: [PATCH] dm ebs: drop dirty bufio state when discarding blocks

[Mikulas Patocka <mpatocka@redhat.com>]

On Wed, 3 Jun 2026, Samuel Moelius wrote:

> dm-ebs can discard a block while a dirty dm-bufio buffer for the
> same block is still cached.  If that buffer is later written back,
> stale data can be written over the discarded state.
> 
> That resurrects data that userspace explicitly discarded and breaks
> the expected discard semantics of the target.
> 
> Invalidate or clean the matching bufio state when processing discards
> so that old dirty buffers cannot be written back after the discard.
> 
> Assisted-by: Codex:gpt-5.5-cyber-preview
> Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
> ---
>  drivers/md/dm-ebs-target.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/md/dm-ebs-target.c b/drivers/md/dm-ebs-target.c
> index 1e52bde48b91..9fece535a0b3 100644
> --- a/drivers/md/dm-ebs-target.c
> +++ b/drivers/md/dm-ebs-target.c
> @@ -212,8 +212,12 @@ static void __ebs_process_bios(struct work_struct *ws)
>  			write = true;
>  			r = __ebs_rw_bio(ec, REQ_OP_WRITE, bio);
>  		} else if (bio_op(bio) == REQ_OP_DISCARD) {
> -			__ebs_forget_bio(ec, bio);
> -			r = __ebs_discard_bio(ec, bio);
> +			if (write)
> +				r = dm_bufio_write_dirty_buffers(ec->bufio);
> +			if (!r) {
> +				__ebs_forget_bio(ec, bio);
> +				r = __ebs_discard_bio(ec, bio);
> +			}
>  		}
>  
>  		if (r < 0)

Hi

There's a bug in the patch - when the "write" variable is false, "r" is 
equal to -EIO, thus __ebs_forget_bio and __ebs_discard_bio is skipped at 
all.

If dm_bufio_write_dirty_buffers returns non-zero, the error is erroneously 
reported on the discard bio (that is innocent) and not on the write bios 
that triggered the error.

Mikulas