Re: [PATCHv3 5/6] block/bounce: count bytes instead of sectors

From: Keith Busch <kbusch@kernel.org>
To: Christoph Hellwig <hch@lst.de>
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>,
	bvanassche@acm.org, damien.lemoal@opensource.wdc.com,
	ebiggers@kernel.org
Subject: Re: [PATCHv3 5/6] block/bounce: count bytes instead of sectors
Date: Wed, 25 May 2022 08:17:29 -0600	[thread overview]
Message-ID: <Yo46eYw1oyq/h5BN@kbusch-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <Yo44R5lhdNkZPGjF@kbusch-mbp.dhcp.thefacebook.com>

On Wed, May 25, 2022 at 08:08:07AM -0600, Keith Busch wrote:
> On Tue, May 24, 2022 at 08:09:21AM +0200, Christoph Hellwig wrote:
> > >  	bio_for_each_segment(from, *bio_orig, iter) {
> > >  		if (i++ < BIO_MAX_VECS)
> > > -			sectors += from.bv_len >> 9;
> > > +			bytes += from.bv_len;
> > >  		if (PageHighMem(from.bv_page))
> > >  			bounce = true;
> > >  	}
> > >  	if (!bounce)
> > >  		return;
> > >  
> > > +	sectors = ALIGN_DOWN(bytes, queue_logical_block_size(q)) >> 9;
> > 
> > Same comment about SECTOR_SHIFT and a comment here.  That being said,
> > why do we even align here?  Shouldn't bytes always be setor aligned here
> > and this should be a WARN_ON or other sanity check?  Probably the same
> > for the previous patch.
> 
> Yes, the total bytes should be sector aligned.
> 
> I'm not exactly sure why we're counting it this way, though. We could just skip
> the iterative addition and get the total from bio->bi_iter.bi_size. Unless
> bio_orig has more segments that BIO_MAX_VECS, which doesn't seem possible.

Oh, there's a comment explaining the original can have more than BIO_MAX_VECS,
so the ALIGN_DOWN is necessary to ensure a logical block sized split.