Re: bio segment constraints

From: Bart Van Assche <bvanassche@acm.org>
To: Christoph Hellwig <hch@infradead.org>, Keith Busch <kbusch@kernel.org>
Cc: Sean Anderson <seanga2@gmail.com>, Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org,
	Miquel Raynal <miquel.raynal@bootlin.com>,
	Richard Weinberger <richard@nod.at>,
	Vignesh Raghavendra <vigneshr@ti.com>,
	linux-mtd@lists.infradead.org,
	Zhihao Cheng <chengzhihao1@huawei.com>
Subject: Re: bio segment constraints
Date: Mon, 7 Apr 2025 08:52:13 -0700	[thread overview]
Message-ID: <e65d2884-a021-44e2-b3cc-9e9badad56e7@acm.org> (raw)
In-Reply-To: <Z_PaW2QS3OXTXHSO@infradead.org>

On 4/7/25 6:59 AM, Christoph Hellwig wrote:
> On Mon, Apr 07, 2025 at 02:46:32PM +0100, Keith Busch wrote:
>> O_DIRECT only requires each user iovec be a multiple of the logical
>> block size with the address aligned to the dma_alignment. If the
>> dma_alignment is smaller than the logical block size, then this could
>> create bvec segments that are smaller. For nvme where we have 4-byte
>> dma alignment, you could have the first segment be the last 4 bytes of a
>> page, then the remaing 508 bytes from a different page in the next
>> segment.
> 
> Oh, right - with a smaller dma alignment this can actually happen.

Some time ago I added src/discontiguous-io.cpp to the blktests project
to trigger this scenario. This test program submits an SG_IO request
with multiple 4 byte segments. Maybe this test program should be
modified such that it uses O_DIRECT instead of SG_IO.

Bart.

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