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

[John Garry]

On 16/01/2025 12:59, Mikulas Patocka wrote:
> > > 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
> Yes - that's right. I suggest that you verify that the atomic flag is
> really passed through the dm-raid1.c and dm-io.c stack. Add a printk that
> tests if REQ_ATOMIC is set to the function do_region in dm-io.c just
> before "submit_bio(bio)".
>
> Alternatively, you can use blktrace to test if the REQ_ATOMIC is passed
> through correctly.

Yes, it is passed ok.

JFYI, I can also verify proper atomic write functionality on /dev/dmX 
with fio in verify mode.

Thanks,
John