Re: [PATCH v2] dm ebs: drop dirty bufio state when discarding blocks

[Mikulas Patocka <mpatocka@redhat.com>]

Hi

I will accept this (and the other patches) after the upcoming merge window 
ends.

Mikulas


On Mon, 8 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.
> 
> Flush pending dirty buffers before processing a following discard, then
> forget the matching bufio state and issue the discard.  Keep writeback
> errors separate from the discard result so a flush failure is reported
> to the write bios that dirtied the buffers, while the discard bio
> reports only the discard operation's own status.
> 
> Assisted-by: Codex:gpt-5.5-cyber-preview
> Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
> ---
> Changes in v2
>   - fix how dirty bios are handled and how errors are reported
> 
>  drivers/md/dm-ebs-target.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/md/dm-ebs-target.c b/drivers/md/dm-ebs-target.c
> index 1e52bde48b91..5bdd1bf6c206 100644
> --- a/drivers/md/dm-ebs-target.c
> +++ b/drivers/md/dm-ebs-target.c
> @@ -176,8 +176,8 @@ static void __ebs_forget_bio(struct ebs_c *ec, struct bio *bio)
>  /* Worker function to process incoming bios. */
>  static void __ebs_process_bios(struct work_struct *ws)
>  {
> -	int r;
> -	bool write = false;
> +	int r, rr, write_r = 0;
> +	bool dirty = false;
>  	sector_t block1, block2;
>  	struct ebs_c *ec = container_of(ws, struct ebs_c, ws);
>  	struct bio *bio;
> @@ -209,9 +209,15 @@ static void __ebs_process_bios(struct work_struct *ws)
>  		if (bio_op(bio) == REQ_OP_READ)
>  			r = __ebs_rw_bio(ec, REQ_OP_READ, bio);
>  		else if (bio_op(bio) == REQ_OP_WRITE) {
> -			write = true;
>  			r = __ebs_rw_bio(ec, REQ_OP_WRITE, bio);
> +			dirty = true;
>  		} else if (bio_op(bio) == REQ_OP_DISCARD) {
> +			if (dirty) {
> +				rr = dm_bufio_write_dirty_buffers(ec->bufio);
> +				dirty = false;
> +				if (rr && !write_r)
> +					write_r = rr;
> +			}
>  			__ebs_forget_bio(ec, bio);
>  			r = __ebs_discard_bio(ec, bio);
>  		}
> @@ -224,11 +230,15 @@ static void __ebs_process_bios(struct work_struct *ws)
>  	 * We write dirty buffers after processing I/O on them
>  	 * but before we endio thus addressing REQ_FUA/REQ_SYNC.
>  	 */
> -	r = write ? dm_bufio_write_dirty_buffers(ec->bufio) : 0;
> +	if (dirty) {
> +		r = dm_bufio_write_dirty_buffers(ec->bufio);
> +		if (r && !write_r)
> +			write_r = r;
> +	}
>  
>  	while ((bio = bio_list_pop(&bios))) {
>  		/* Any other request is endioed. */
> -		if (unlikely(r && bio_op(bio) == REQ_OP_WRITE))
> +		if (unlikely(write_r && bio_op(bio) == REQ_OP_WRITE))
>  			bio_io_error(bio);
>  		else
>  			bio_endio(bio);
> -- 
> 2.43.0
>