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

[Samuel Moelius]

On Mon, Jun 8, 2026 at 10:16 AM Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
>
>
> 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@xxxxxxxxxxxxxxx>
> > ---
> >  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.

v2 forthcoming.