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

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Message-ID: <e2be0a8c-3b97-f75c-362e-2174340b1b2c@redhat.com>
Date: Thu, 16 Jan 2025 13:59:35 +0100 (CET)
From: Mikulas Patocka <mpatocka@...hat.com>
To: John Garry <john.g.garry@...cle.com>
cc: Mike Snitzer <snitzer@...merspace.com>, axboe@...nel.dk, agk@...hat.com, 
    hch@....de, martin.petersen@...cle.com, linux-block@...r.kernel.org, 
    dm-devel@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 0/5] device mapper atomic write support



On Thu, 16 Jan 2025, John Garry wrote:

> On 07/01/2025 17:13, Mikulas Patocka wrote:
> > 
> > 
> > On Mon, 6 Jan 2025, John Garry wrote:
> > 
> > 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

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.

Mikulas