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Date: Tue, 29 Apr 2025 14:15:29 +0200
From: Christoph Hellwig <hch@lst.de>
To: Yu Kuai <yukuai1@huaweicloud.com>
Cc: Keith Busch <kbusch@kernel.org>, Christoph Hellwig <hch@lst.de>,
	Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
	Hannes Reinecke <hare@suse.de>, "yukuai (C)" <yukuai3@huawei.com>
Subject: Re: [PATCH 4/5] brd: split I/O at page boundaries
Message-ID: <20250429121529.GB12411@lst.de>
References: <20250428141014.2360063-1-hch@lst.de> <20250428141014.2360063-5-hch@lst.de> <aA_Dyp97AIAqJ70G@kbusch-mbp.dhcp.thefacebook.com> <221bce43-83b7-b5ac-c6d2-ded23158dd06@huaweicloud.com>
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On Tue, Apr 29, 2025 at 09:38:28AM +0800, Yu Kuai wrote:
> Hi,
>
> 在 2025/04/29 2:07, Keith Busch 写道:
>> On Mon, Apr 28, 2025 at 07:09:50AM -0700, Christoph Hellwig wrote:
>>> A lot of complexity in brd stems from the fact that it tries to handle
>>> I/O spanning two backing pages.  Instead limit the size of a single
>>> bvec iteration so that it never crosses a page boundary and remove all
>>> the now unneeded code.
>>
>> Doesn't bio_for_each_segment() already limit bvecs on page boundaries?
>> You'd need to use bio_for_each_bvec() to get multi-page bvecs.
>
> I think it only limit bvecs on page boundaries on the issue side, not
> disk side.
>
> For example, if user issue an IO (2k + 4k), will bio_for_each_segment()
> split this IO into (2k + 2k) and (4k + 2k), I do not test yet, but I
> think the answer is no.

Exactly.  I got this wrong with zram, where it only triggers with larger
than 4k page sizes, and I got this wrong here on my first attempt as
well.  Fortunately testing found it quickly.  I thought the comment and
commit message document the issue well enough, but I'm open to better
wording.