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From: Christoph Hellwig <hch@lst.de>
To: Ming Lei <ming.lei@canonical.com>
Cc: Christoph Hellwig <hch@lst.de>, Al Viro <viro@zeniv.linux.org.uk>,
	Jens Axboe <axboe@fb.com>,
	Nicholas Bellinger <nab@linux-iscsi.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	target-devel@vger.kernel.org
Subject: Re: [PATCH 3/3] target: use vfs_bvec_read/write
Date: Mon, 26 Jan 2015 17:59:12 +0100	[thread overview]
Message-ID: <20150126165912.GA13669@lst.de> (raw)
In-Reply-To: <CACVXFVNMLQAVn3fCXaRG1JoRZ8Y_b3kAOjWh0yicrO9VMJu3uw@mail.gmail.com>

On Mon, Jan 26, 2015 at 10:02:30AM +0800, Ming Lei wrote:
> >> Sorry, I have one question: I understand one bvec should only cover
> >> one page, but
> >> one sg may cover lots of pages, so could ITER_BVEC handle that correctly?
> >
> > Each scatterlist entry only contains a single page, which is returned
> 
> I mean scatterlist does not guarantee that, and one sg entry often
> contains lots of pages, which DMA/bus address is continuous.

But we still get away with using page_address(), so from the VM
point of view that matters here it's the same as a single high order
page.

> > by sg_page(sg).  The existing code already relies on it because it
> > kmaps that page.
> 
> If the existing target code path can guarantee that one sg entry
> only contains one page, it should be better to use bio_vec explicitly
> instead of scatterlist.

While it allocates a single page per SGL entry (see target_alloc_sgl),
it uses the scatterlist to allow drivers to dma map it for hardware
access, so a scatterlist seems useful here.

  reply	other threads:[~2015-01-26 16:59 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-01-18 15:07 switch loop and target to use ITER_BVEC iov_iter Christoph Hellwig
2015-01-18 15:07 ` [PATCH 1/3] fs: add vfs_bvec_{read,write} helpers Christoph Hellwig
2015-01-23  6:00   ` Al Viro
2015-01-18 15:07 ` [PATCH 2/3] loop: convert to vfs_bvec_write Christoph Hellwig
2015-01-18 15:07 ` [PATCH 3/3] target: use vfs_bvec_read/write Christoph Hellwig
2015-01-18 15:37   ` Sagi Grimberg
2015-01-20 23:32   ` Nicholas A. Bellinger
2015-01-23 14:08   ` Ming Lei
2015-01-25 13:43     ` Christoph Hellwig
2015-01-26  2:02       ` Ming Lei
2015-01-26 16:59         ` Christoph Hellwig [this message]
2015-01-27  5:14           ` Ming Lei
2015-01-22  4:11 ` switch loop and target to use ITER_BVEC iov_iter Ming Lei
2015-01-25 13:45   ` Christoph Hellwig

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