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>

1 files changed, 22 insertions, 3 deletions

diff --git a/fs/jbd/commit.c b/fs/jbd/commit.c
index 86b39b1..11bb11f 100644
--- a/fs/jbd/commit.c
+++ b/fs/jbd/commit.c
@@ -162,8 +162,17 @@ static void journal_do_submit_data(struct buffer_head **wbuf, int bufs,
 
 	for (i = 0; i < bufs; i++) {
 		wbuf[i]->b_end_io = end_buffer_write_sync;
-		/* We use-up our safety reference in submit_bh() */
-		submit_bh(write_op, wbuf[i]);
+		/*
+		 * Here we write back pagecache data that may be mmaped. Since
+		 * we cannot afford to clean the page and set PageWriteback
+		 * here due to lock ordering (page lock ranks above transaction
+		 * start), the data can change while IO is in flight. Tell the
+		 * block layer it should bounce the bio pages if stable data
+		 * during write is required.
+		 *
+		 * We use up our safety reference in submit_bh().
+		 */
+		_submit_bh(write_op, wbuf[i], 1 << BIO_SNAP_STABLE);
 	}
 }
 
@@ -667,7 +676,17 @@ start_journal_io:
 				clear_buffer_dirty(bh);
 				set_buffer_uptodate(bh);
 				bh->b_end_io = journal_end_buffer_io_sync;
-				submit_bh(write_op, bh);
+				/*
+				 * In data=journal mode, here we can end up
+				 * writing pagecache data that might be
+				 * mmapped. Since we can't afford to clean the
+				 * page and set PageWriteback (see the comment
+				 * near the other use of _submit_bh()), the
+				 * data can change while the write is in
+				 * flight.  Tell the block layer to bounce the
+				 * bio pages if stable pages are required.
+				 */
+				_submit_bh(write_op, bh, 1 << BIO_SNAP_STABLE);
 			}
 			cond_resched();