mm: make snapshotting pages for stable writes a per-bio operation

Walking a bio's page mappings has proved problematic, so create a new
bio flag to indicate that a bio's data needs to be snapshotted in order
to guarantee stable pages during writeback.  Next, for the one user
(ext3/jbd) of snapshotting, hook all the places where writes can be
initiated without PG_writeback set, and set BIO_SNAP_STABLE there.

We must also flag journal "metadata" bios for stable writeout, since
file data can be written through the journal.  Finally, the
MS_SNAP_STABLE mount flag (only used by ext3) is now superfluous, so get
rid of it.

[akpm@linux-foundation.org: rename _submit_bh()'s `flags' to `bio_flags', delobotomize the _submit_bh declaration]
[akpm@linux-foundation.org: teeny cleanup]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Artem Bityutskiy <dedekind1@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/bounce.c

@@ -181,32 +181,13 @@ static void bounce_end_io_read_isa(struct bio *bio, int err)
 #ifdef CONFIG_NEED_BOUNCE_POOL
 static int must_snapshot_stable_pages(struct request_queue *q, struct bio *bio)
 {
-	struct page *page;
-	struct backing_dev_info *bdi;
-	struct address_space *mapping;
-	struct bio_vec *from;
-	int i;
-
 	if (bio_data_dir(bio) != WRITE)
 		return 0;
 
 	if (!bdi_cap_stable_pages_required(&q->backing_dev_info))
 		return 0;
 
-	/*
-	 * Based on the first page that has a valid mapping, decide whether or
-	 * not we have to employ bounce buffering to guarantee stable pages.
-	 */
-	bio_for_each_segment(from, bio, i) {
-		page = from->bv_page;
-		mapping = page_mapping(page);
-		if (!mapping)
-			continue;
-		bdi = mapping->backing_dev_info;
-		return mapping->host->i_sb->s_flags & MS_SNAP_STABLE;
-	}
-
-	return 0;
+	return test_bit(BIO_SNAP_STABLE, &bio->bi_flags);
 }
 #else
 static int must_snapshot_stable_pages(struct request_queue *q, struct bio *bio)