Re: [PATCH RFC 0/5] device mapper atomic write support

[John Garry]

On 07/01/2025 17:13, Mikulas Patocka wrote:
> On Mon, 6 Jan 2025, John Garry wrote:
>
> > On 06/01/2025 17:26, Mike Snitzer wrote:
> > > On Mon, Jan 06, 2025 at 12:41:14PM +0000, John Garry wrote:
> > > > This series introduces initial device mapper atomic write support.
> > > >
> > > > Since we already support stacking atomic writes limits, it's quite
> > > > straightforward to support.
> > > >
> > > > Only dm-linear is supported for now, but other personalities could
> > > > be supported.
> > > >
> > > > Patch #1 is a proper fix, but the rest of the series is RFC - this is
> > > > because I have not fully tested and we are close to the end of this
> > > > development cycle.
> > > In general, looks reasonable.  But I would prefer to see atomic write
> > > support added to dm-striped as well.  Not that I have some need, but
> > > because it will help verify the correctness of the general stacking
> > > code changes (in both block and DM core).
> >
> > That should be fine. We already have md raid0 support working (for atomic
> > writes), so I would expect much of the required support is already available.
>
> BTW. could it be possible to add dm-mirror support as well? dm-mirror is
> used when the user moves the logical volume to another physical volume, so
> it would be nice if this worked without resulting in not-supported errors.
>
> dm-mirror uses dm-io to perform the writes on multiple mirror legs (see
> the function do_write() -> dm_io()), I looked at the code and it seems
> that the support for atomic writes in dm-mirror and dm-io would be
> straightforward.

I tried this out, and it seems to work ok.

However, I need to set DM_TARGET_ATOMIC_WRITES in the 
mirror_target.features member, like:

diff --git a/drivers/md/dm-raid1.c b/drivers/md/dm-raid1.c
index 9511dae5b556..913a92c55904 100644
--- a/drivers/md/dm-raid1.c
+++ b/drivers/md/dm-raid1.c
@@ -1485,6 +1485,7 @@ static struct target_type mirror_target = {
	.name    = "mirror",
	.version = {1, 14, 0},
	.module  = THIS_MODULE,
+	.features = DM_TARGET_ATOMIC_WRITES,
	.ctr     = mirror_ctr,
	.dtr     = mirror_dtr,
	.map     = mirror_map,


Is this the right thing to do? I ask, as none of the other DM_TARGET* 
flags are set already, which makes me suspicious.

Thanks,
John