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Date: Thu, 21 Dec 2023 13:19:25 +0100
From: Christoph Hellwig <hch@lst.de>
To: John Garry <john.g.garry@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>, "Darrick J. Wong" <djwong@kernel.org>,
	axboe@kernel.dk, kbusch@kernel.org, sagi@grimberg.me,
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Subject: Re: [PATCH v2 00/16] block atomic writes
Message-ID: <20231221121925.GB17956@lst.de>
References: <b8b0a9d7-88d2-45a9-877a-ecc5e0f1e645@oracle.com> <20231213154409.GA7724@lst.de> <c729b03c-b1d1-4458-9983-113f8cd752cd@oracle.com> <20231219051456.GB3964019@frogsfrogsfrogs> <20231219052121.GA338@lst.de> <76c85021-dd9e-49e3-80e3-25a17c7ca455@oracle.com> <20231219151759.GA4468@lst.de> <fff50006-ccd2-4944-ba32-84cbb2dbd1f4@oracle.com> <20231221065031.GA25778@lst.de> <b60e39ce-04bf-4ff9-8879-d9e0cf5d84bd@oracle.com>
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On Thu, Dec 21, 2023 at 09:49:35AM +0000, John Garry wrote:
> I noticed the NVMe patch to stop always setting virt boundary (thanks), but 
> I am struggling for the wording for iovecs rules. I'd like to reuse 
> iov_iter_is_aligned() to enforce any such rule.
>
> I am thinking:
> - ubuf / iovecs need to be PAGE-aligned
> - each iovec needs to be length of multiple of PAGE_SIZE
>
> But that does not work for total length < PAGE_SIZE.
>
> 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,
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.