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

From: Keith Busch <kbusch@kernel.org>
To: Eric Biggers <ebiggers@kernel.org>
Cc: Keith Busch <kbusch@fb.com>,
	linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	axboe@kernel.dk, Kernel Team <Kernel-team@fb.com>,
	hch@lst.de, bvanassche@acm.org, damien.lemoal@opensource.wdc.com
Subject: Re: [PATCHv2 3/3] block: relax direct io memory alignment
Date: Wed, 18 May 2022 22:56:04 -0600	[thread overview]
Message-ID: <YoXN5CpSGGe7+OJs@kbusch-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <YoW5Iy+Vbk4Rv3zT@sol.localdomain>

On Wed, May 18, 2022 at 08:27:31PM -0700, Eric Biggers wrote:
> 
> So the bio ends up with a total length that is a multiple of the logical block
> size, but the lengths of the individual bvecs in the bio are *not* necessarily
> multiples of the logical block size.  That's the problem.

I'm surely missing something here. I know the bvecs are not necessarily lbs
aligned, but why does that matter? Is there some driver that can only take
exactly 1 bvec, but allows it to be unaligned? If so, we could take the segment
queue limit into account, but I am not sure that we need to.

> Note, there's also lots of code that assumes that bio_vec::bv_len is a multiple
> of 512.  

Could you point me to some examples?

> That was implied by it being a multiple of the logical block size.  But
> the DMA alignment can be much lower, like 8 bytes (see nvme_set_queue_limits()).

That's the driver this was tested on, though I just changed it to 4 bytes for
5.19.