linux-kernel - Did immutable bvecs accidentally break stable page writes?

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Date:	Thu, 19 Sep 2013 10:32:50 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>,
	Kent Overstreet <koverstreet@...gle.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Did immutable bvecs accidentally break stable page writes?

Commit ffecfd1a (block: optionally snapshot page contents to provide
stable pages during write) uses bounce buffers for stable page writes in
jbd and ext3. Simplistically, __blk_queue_bounce takes a force parameter
that is used when pages must be snapshot.

Commit 6bc454d1 (bounce: Refactor __blk_queue_bounce to not use
bi_io_vec) refactored __blk_queue_bounce and now the start of the
function looks like this

static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
			       mempool_t *pool, int force)
{
	struct bio *bio;
	int rw = bio_data_dir(*bio_orig);
	struct bio_vec *to, *from;
	unsigned i;

	bio_for_each_segment(from, *bio_orig, i)
		if (page_to_pfn(from->bv_page) > queue_bounce_pfn(q))
			goto bounce;

	return;
bounce:
	bio = bio_clone_bioset(*bio_orig, GFP_NOIO, fs_bio_set);

	bio_for_each_segment_all(to, bio, i) {
		struct page *page = to->bv_page;

		if (page_to_pfn(page) <= queue_bounce_pfn(q) && !force)
			continue;

Note that the first bio_for_each_segment is completely ignoring the
force parameter and hence snapshotting. This is particularly problematic
for ext3 which forces the use of MS_SNAP_STABLE.

I have not actually reproduced any problem, this is just code inspection
but it looks like commit 6bc454d1 broke ext3. Kent, why was the force
paramter ignored in that commit?

-- 
Mel Gorman
SUSE Labs
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