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Date:	Thu, 19 Sep 2013 10:40:30 +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: Re: Did immutable bvecs accidentally break stable page writes?

On Thu, Sep 19, 2013 at 10:32:50AM +0100, Mel Gorman wrote:
> 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.
> 

Which of course is no longer a problem for ext3 after commit 71368511
("mm: make snapshotting pages for stable writes a per-bio operation). The
folly of looking at a commit in isolation! I'm still curious why the force
parameter is ignored when stable writes are required though.

-- 
Mel Gorman
SUSE Labs
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