linux-kernel - Re: [PATCH RFC 02/16] fs/bdev: Add atomic write support info to statx

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Message-ID: <20230504224033.GJ3223426@dread.disaster.area>
Date:   Fri, 5 May 2023 08:40:33 +1000
From:   Dave Chinner <david@...morbit.com>
To:     John Garry <john.g.garry@...cle.com>
Cc:     axboe@...nel.dk, kbusch@...nel.org, hch@....de, sagi@...mberg.me,
        martin.petersen@...cle.com, djwong@...nel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org, dchinner@...hat.com,
        jejb@...ux.ibm.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-scsi@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org, paul@...l-moore.com,
        jmorris@...ei.org, serge@...lyn.com,
        Prasad Singamsetty <prasad.singamsetty@...cle.com>
Subject: Re: [PATCH RFC 02/16] fs/bdev: Add atomic write support info to statx

On Thu, May 04, 2023 at 09:45:50AM +0100, John Garry wrote:
> On 03/05/2023 22:58, Dave Chinner wrote:
> > Is there a statx() man
> > page update for this addition?
> 
> No, not yet. Is it normally expected to provide a proposed man page update
> in parallel? Or somewhat later, when the kernel API change has some
> appreciable level of agreement?

Normally we ask for man page updates to be presented at the same
time, as the man page defines the user interface that is being
implemented. In this case, we need updates for the pwritev2() man
page to document RWF_ATOMIC semantics, and the statx() man page to
document what the variables being exposed mean w.r.t. RWF_ATOMIC.

The pwritev2() man page is probably the most important one right now
- it needs to explain the guarantees that RWF_ATOMIC is supposed to
provide w.r.t. data integrity, IO ordering, persistence, etc.
Indeed, it will need to explain exactly how this "multi-atomic-unit
mulit-bio non-atomic RWF_ATOMIC" IO thing can be used safely and
reliably, especially w.r.t. IO ordering and persistence guarantees
in the face of crashes and power failures. Not to mention
documenting error conditions specific to RWF_ATOMIC...

It's all well and good to have some implementation, but without
actually defining and documenting the *guarantees* that RWF_ATOMIC
provides userspace it is completely useless for application
developers. And from the perspective of a reviewer, without the
documentation stating what the infrastructure actually guarantees
applications, we can't determine if the implementation being
presented is fit for purpose....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com