On Fri 06-08-10 06:39:29, Wu Fengguang wrote: > On Fri, Aug 06, 2010 at 01:00:16AM +0800, Jan Kara wrote: > > 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. > > 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); > } 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 < MAX_WRITEBACK_PAGES check would be good. > > Actually, this comment probably also applies to your patch where you > > change the queueing logic in writeback_single_inode(), doesn't it? > > Can you elaborate? 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html