linux-kernel - Re: [PATCH v2 00/16] block atomic writes

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Date: Thu, 21 Dec 2023 12:48:24 +0000
From: John Garry <john.g.garry@...cle.com>
To: Christoph Hellwig <hch@....de>
Cc: "Darrick J. Wong" <djwong@...nel.org>, axboe@...nel.dk, kbusch@...nel.org,
        sagi@...mberg.me, jejb@...ux.ibm.com, martin.petersen@...cle.com,
        viro@...iv.linux.org.uk, brauner@...nel.org, dchinner@...hat.com,
        jack@...e.cz, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        tytso@....edu, jbongio@...gle.com, linux-scsi@...r.kernel.org,
        ming.lei@...hat.com, jaswin@...ux.ibm.com, bvanassche@....org
Subject: Re: [PATCH v2 00/16] block atomic writes

On 21/12/2023 12:19, Christoph Hellwig wrote:
>> So then we could have:
>> - ubuf / iovecs need to be PAGE-aligned
>> - each iovec needs to be length of multiple of atomic_write_unit_min. If
>> total length > PAGE_SIZE, each iovec also needs to be a multiple of
>> PAGE_SIZE.
>>
>> I'd rather something simpler. Maybe it's ok.
> If we decided to not support atomic writes on anything setting a virt
> boundary we don't have to care about the alignment of each vector,

ok, I think that alignment is not so important, but we still need to 
consider a minimum length per iovec, such that we will always be able to 
fit a write of length atomic_write_unit_max in a bio.

> and IMHO we should do that as everything else would be a life in
> constant pain.  If we really have a use case for atomic writes on
> consumer NVMe devices we'll just have to limit it to a single iovec.

I'd be more inclined to say that SGL support is compulsory, but I don't 
know if that is limiting atomic writes support to an unsatisfactory 
market segment.