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Message-ID: <9bee0c1c-e657-4201-beb2-f8163bc945c6@oracle.com>
Date: Thu, 21 Dec 2023 13:18:33 +0000
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Subject: Re: [PATCH v2 00/16] block atomic writes
To: Christoph Hellwig <hch@lst.de>
Cc: "Darrick J. Wong" <djwong@kernel.org>, axboe@kernel.dk, kbusch@kernel.org,
        sagi@grimberg.me, jejb@linux.ibm.com, martin.petersen@oracle.com,
        viro@zeniv.linux.org.uk, brauner@kernel.org, dchinner@redhat.com,
        jack@suse.cz, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
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References: <c729b03c-b1d1-4458-9983-113f8cd752cd@oracle.com>
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From: John Garry <john.g.garry@oracle.com>
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On 21/12/2023 12:57, Christoph Hellwig wrote:
> On Thu, Dec 21, 2023 at 12:48:24PM +0000, John Garry wrote:
>>>> - 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.
> 
> I don't think you man a minim length per iovec for that, but a maximum
> number of iovecs instead. 

That would make sense, but I was thinking that if the request queue has 
a limit on segments then we would need to specify a iovec min length.

> For SGL-capable devices that would be
> BIO_MAX_VECS, otherwise 1.

ok, but we would need to advertise that or whatever segment limit. A 
statx field just for that seems a bit inefficient in terms of space.