On Thu 19-09-13 10:40:30, 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. This really looks like a bug to me. Since stable pages are currently required only for drives supporting DIF/DIX noone has likely noticed. But still we should fix that.
Honza PS: As an unrelated note first 8 digits of the commit SHA are not unique for that commit. Time to switch to longer prefixes? :) -- Jan Kara <j...@suse.cz> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/