linux-kernel - Re: [PATCH 21/21] nvme: Support atomic writes

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Date:   Fri, 10 Nov 2023 08:44:45 +0000
From:   John Garry <john.g.garry@...cle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Matthew Wilcox <willy@...radead.org>, axboe@...nel.dk,
        kbusch@...nel.org, sagi@...mberg.me, jejb@...ux.ibm.com,
        martin.petersen@...cle.com, djwong@...nel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chandan.babu@...cle.com, dchinner@...hat.com,
        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-api@...r.kernel.org, Alan Adamson <alan.adamson@...cle.com>
Subject: Re: [PATCH 21/21] nvme: Support atomic writes

On 10/11/2023 06:29, Christoph Hellwig wrote:
> Yes.
> 
>> As for splitting, it is not permitted for atomic writes and only a single
>> bio is permitted to be created per write. Are more integrity checks
>> required?
> I'm more worried about the problem where we accidentally add a split.
> The whole bio merge/split path is convoluted and we had plenty of
> bugs in the past by not looking at all the correct flags or opcodes.

Yes, this is always a concern.

Some thoughts on things which could be done:
- For no merging, ensure request length is a power-of-2 when enqueuing 
to block driver. This is simple but not watertight.
- Create a per-bio checksum when the bio is created for the atomic write 
and ensure integrity when queuing to the block driver
- a new block layer datapath which ensures no merging or splitting, but 
this seems a bit OTT

BTW, on topic of splitting, that NVMe virt boundary is a pain and I hope 
that we could ignore/avoid it for atomic writes.

Thanks,
John