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

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Date:	Thu, 19 Sep 2013 19:38:44 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Kent Overstreet <koverstreet@...gle.com>,
	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 10:40:30AM +0100, Mel Gorman wrote:
> 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.

I'm confused by this (second) statement.  Mel, I think you were arguing that
this is broken because BIO_SNAP_STABLE sets force=1, yet __blk_queue_bounce()
ignores "force", which leads to the pages not being bounced.

So it /is/ a likely regression, and testing confirms that DIF+ext3 is broken.
I believe that there should be a "if(force) goto bounce;" just after the
variable declarations.  I'll go write a patch and check though.

--D
> 
> -- 
> Mel Gorman
> SUSE Labs
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