Re: [PATCHv2 3/3] block: relax direct io memory alignment

From: Christoph Hellwig <hch@lst.de>
To: Keith Busch <kbusch@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Keith Busch <kbusch@fb.com>,
	linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	axboe@kernel.dk, Kernel Team <Kernel-team@fb.com>,
	bvanassche@acm.org, damien.lemoal@opensource.wdc.com
Subject: Re: [PATCHv2 3/3] block: relax direct io memory alignment
Date: Fri, 20 May 2022 08:10:53 +0200	[thread overview]
Message-ID: <20220520061053.GB16557@lst.de> (raw)
In-Reply-To: <YoZPcqDpwSTn/csn@kbusch-mbp>

On Thu, May 19, 2022 at 08:08:50AM -0600, Keith Busch wrote:
> > >  	size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
> > > +	if (size > 0)
> > > +		size = ALIGN_DOWN(size, queue_logical_block_size(q));
> > 
> > So if we do get a size that is not logical block size alignment here,
> > we reduce it to the block size aligned one below.  Why do we do that?
> 
> There are two possibilities:
> 
> In the first case, the number of pages in this iteration exceeds bi_max_vecs.
> Rounding down completes the bio with a block aligned size, and the remainder
> will be picked up for the next bio, or possibly even the current bio if the
> pages are sufficiently physically contiguous.
> 
> The other case is a bad iov. If we're doing __blkdev_direct_IO(), it will error
> out immediately if the rounded size is 0, or the next iteration when the next
> size is rounded to 0. If we're doing the __blkdev_direct_IO_simple(), it will
> error out when it sees the iov hasn't advanced to the end.

Can you please document this with a comment in the code?