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Date: Wed, 9 Nov 2011 05:34:15 -0500
From: Christoph Hellwig <hch@infradead.org>
To: Jan Schmidt <list.btrfs@jan-o-sch.net>
Cc: linux-btrfs <linux-btrfs@vger.kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Chris Mason <chris.mason@oracle.com>, Arne Jansen <sensille@gmx.net>,
        Andrea Gelmini <andrea.gelmini@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [GIT PULL] Btrfs pull request
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On Wed, Nov 09, 2011 at 11:29:31AM +0100, Jan Schmidt wrote:
> Am 09.11.2011 08:48, schrieb Christoph Hellwig:
> >On Tue, Nov 08, 2011 at 08:07:01PM -0500, Chris Mason wrote:
> >>Looks like bio_add_page() is failing and we're getting the enomem from
> >>there.  LVM is only letting us put one page in each bio.
> >
> >Yes, at the moment all bio based DM targets only allow single page I/O.
> 
> Wait. If I got that correctly, each bio_add_page needs special
> ENOMEM treatment (assuming the target could always be a device
> mapper target), right?

Each bio_add_page caller needs to expect it can't add more than a
page worth of data.  If you look at callers what write large amounts
of data (XFS, mpage code) you'll always see a pattern of:

	bio = bio_alloc();

	while (bytes_left) {
		len = min(bytes_left, PAGE_SIZE);
		bytes = bio_add_page(io, ..., len);
		if (bytes < len) {
			submit_bio(bio);
			bio = bio_alloc();
		}
		update_indices();
	}
	submit_bio(bio);