writeback_inodes_wb()/__writeback_inodes_sb() are not aggressive in that
they only populate b_io when necessary at entrance time. When the queued
set of inodes are all synced, they just return, possibly with
wbc.nr_to_write > 0.

For kupdate and background writeback, there may be more eligible inodes
sitting in b_dirty when the current set of b_io inodes are completed. So
it is necessary to try another round of writeback as long as we made some
progress in this round. When there are no more eligible inodes, no more
inodes will be enqueued in queue_io(), hence nothing could/will be
synced and we may safely bail.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/fs-writeback.c |   19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

--- linux-next.orig/fs/fs-writeback.c	2010-08-05 23:30:27.000000000 +0800
+++ linux-next/fs/fs-writeback.c	2010-08-05 23:30:45.000000000 +0800
@@ -654,20 +654,23 @@ static long wb_writeback(struct bdi_writ
 		wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
 
 		/*
-		 * If we consumed everything, see if we have more
+		 * Did we write something? Try for more
+		 *
+		 * This is needed _before_ the b_more_io test because the
+		 * background writeback moves inodes to b_io and works on
+		 * them in batches (in order to sync old pages first).  The
+		 * completion of the current batch does not necessarily mean
+		 * the overall work is done.
 		 */
-		if (wbc.nr_to_write <= 0)
+		if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
 			continue;
+
 		/*
-		 * Didn't write everything and we don't have more IO, bail
+		 * Nothing written and no more inodes for IO, bail
 		 */
 		if (list_empty(&wb->b_more_io))
 			break;
-		/*
-		 * Did we write something? Try for more
-		 */
-		if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
-			continue;
+
 		/*
 		 * Nothing written. Wait for some inode to
 		 * become available for writeback. Otherwise


--

[ message continues ]

Well, this applies generally to any writeback, not just a background one
right? Whenever we process all inodes from b_io list and move them
somewhere else than b_more_io, then this applies. Some new dirty data could
have arrived while we were doing the write... I'm just afraid that in some
pathological cases this could result in bad writeback pattern - like if
there is some process which manages to dirty just a few pages while we are
doing writeout, this looping could result in writing just a few pages in
each round which is bad for fragmentation etc.
  Actually, this comment probably also applies to your patch where you
change the queueing logic in writeback_single_inode(), doesn't it?

-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR
--

[ message continues ]

Right. Only that it is a requirement for background writeback.

Such inodes will be redirty_tail()ed here:

                if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
                        /*
                         * We didn't write back all the pages.  nfs_writepages()
                         * sometimes bales out without doing anything.
                         */
                        inode->i_state |= I_DIRTY_PAGES;
                        if (wbc->nr_to_write <= 0) {
                                /*
                                 * slice used up: queue for next turn
                                 */
                                requeue_io(inode);
                        } else {
                                /*
                                 * Writeback blocked by something other than
                                 * congestion. Delay the inode for some time to
                                 * avoid spinning on the CPU (100% iowait)
                                 * retrying writeback of the dirty page/inode
                                 * that cannot be performed immediately.
                                 */
                                redirty_tail(inode);

Can you elaborate?
 
Thanks,
--

[ message continues ]

Yes. And then, when there are no inodes in b_more_io, they get queued
again for writeback. So for non-background WB_SYNC_NONE writeback we can
just write a few pages over and over again... Oh, ok we won't because of
my start_time fix I suppose. Maybe a comment about this by the nr_to_write
  Sorry, my comment only applies to this particular patch. In your change
to writeback_single_inode() you requeue_io() only if nr_to_write <= 0.

								Honza
--

[ message continues ]