linux-kernel - Re: [PATCH 1/4] iomap: Lift blocksize restriction on atomic writes

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Message-ID: <cc3824af-0eb7-4bfc-ad81-3c5cebd1c5c3@oracle.com>
Date: Mon, 20 Jan 2025 08:29:55 +0000
From: John Garry <john.g.garry@...cle.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Dave Chinner <david@...morbit.com>, brauner@...nel.org, cem@...nel.org,
        dchinner@...hat.com, hch@....de, ritesh.list@...il.com,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, martin.petersen@...cle.com
Subject: Re: [PATCH 1/4] iomap: Lift blocksize restriction on atomic writes

On 17/01/2025 18:29, Darrick J. Wong wrote:
>>> Ok, let's do that then.  Just to be clear -- for any RWF_ATOMIC direct
>>> write that's correctly aligned and targets a single mapping in the
>>> correct state, we can build the untorn bio and submit it.  For
>>> everything else, prealloc some post EOF blocks, write them there, and
>>> exchange-range them.
>> I have some doubt about this, but I may be misunderstanding the concept:
>>
>> So is there any guarantee that what we write into is aligned (after the
>> exchange-range routine)? If not, surely every subsequent write with
>> RWF_ATOMIC to that logical range will require this exchange-range routine
>> until we get something aligned (and correct granularity) - correct?
> Correct, you'd still need forcealign to make sure that the new
> allocations for exchange-range are aligned to awumin.

Sure, but I think that if we align sb_agblocks to the bdev atomic write 
limits at mkfs time and also set extszhint appropriately, then 
probability of desired alignment and granularity is appreciably higher.

Thanks,
John