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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