From: Wu Fengguang Subject: Re: [PATCH] writeback: reset inode dirty time when adding it back to empty s_dirty list Date: Wed, 25 Mar 2009 22:16:18 +0800 Message-ID: <20090325141618.GA5684@localhost> References: <20090324104657.6907b19e@tleilax.poochiereds.net> <20090325012829.GA7506@localhost> <20090324221528.2bb7c50b@tleilax.poochiereds.net> <20090325025037.GA17374@localhost> <20090325075110.028f0d1d@tleilax.poochiereds.net> <20090325121742.GA22869@localhost> <20090325091325.17c997fd@tleilax.poochiereds.net> <49CA2F41.8030804@themaw.net> <49CA33E7.6090309@themaw.net> <20090325100049.0cc4de87@tleilax.poochiereds.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Ian Kent , Dave Chinner , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "jens.axboe@oracle.com" , "akpm@linux-foundation.org" , "hch@infradead.org" , "linux-nfs@vger.kernel.org" To: Jeff Layton Return-path: In-Reply-To: <20090325100049.0cc4de87@tleilax.poochiereds.net> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Wed, Mar 25, 2009 at 10:00:49PM +0800, Jeff Layton wrote: > On Wed, 25 Mar 2009 22:38:47 +0900 > Ian Kent wrote: > > > Ian Kent wrote: > > > Jeff Layton wrote: > > >> On Wed, 25 Mar 2009 20:17:43 +0800 > > >> Wu Fengguang wrote: > > >> > > >>> On Wed, Mar 25, 2009 at 07:51:10PM +0800, Jeff Layton wrote: > > >>>> On Wed, 25 Mar 2009 10:50:37 +0800 > > >>>> Wu Fengguang wrote: > > >>>> > > >>>>>> Given the right situation though (or maybe the right filesystem), it's > > >>>>>> not too hard to imagine this problem occurring even in current mainline > > >>>>>> code with an inode that's frequently being redirtied. > > >>>>> My reasoning with recent kernel is: for kupdate, s_dirty enqueues only > > >>>>> happen in __mark_inode_dirty() and redirty_tail(). Newly dirtied > > >>>>> inodes will be parked in s_dirty for 30s. During which time the > > >>>>> actively being-redirtied inodes, if their dirtied_when is an old stuck > > >>>>> value, will be retried for writeback and then re-inserted into a > > >>>>> non-empty s_dirty queue and have their dirtied_when refreshed. > > >>>>> > > >>>> Doesn't that assume that there are new inodes that are being dirtied? > > >>>> If you only have the same inodes being redirtied and never any new > > >>>> ones, the problem still occurs, right? > > >>> Yes. But will a production server run months without making one single > > >>> new dirtied inode? (Just out of curiosity. Not that I'm not willing to > > >>> fix this possible issue.:) > > >>> > > >> Yes. It's not that the box will run that long without creating a > > >> single new dirtied inode, but rather that it won't necessarily create > > >> one on all of its mounts. It's often the case that someone has a > > >> mountpoint for a dedicated purpose. > > >> > > >> Consider a host that has a mountpoint that contains logfiles that are > > >> being heavily written. There's nothing that says that they must rotate > > >> those logs over a particular period (assuming the fs has enough space, > > >> etc). If the same ones are constantly being redirtied and no new > > >> ones are created, then I think this problem can easily happen. > > >> > > >>>>>>> ...I see no obvious reasons against unconditionally resetting dirtied_when. > > >>>>>>> > > >>>>>>> (a) Delaying an inode's writeback for 30s maybe too long - its blocking > > >>>>>>> condition may well go away within 1s. (b) And it would be very undesirable > > >>>>>>> if one big file is repeatedly redirtied hence its writeback being > > >>>>>>> delayed considerably. > > >>>>>>> > > >>>>>>> However, redirty_tail() currently only tries to speedup writeback-after-redirty > > >>>>>>> in a _best effort_ way. It at best partially hides the above issues, > > >>>>>>> if there are any. In particular, if (b) is possible, the bug should > > >>>>>>> already show up at least in some situations. > > >>>>>>> > > >>>>>>> For XFS, immediately sync of redirtied inode is actually discouraged: > > >>>>>>> > > >>>>>>> http://lkml.org/lkml/2008/1/16/491 > > >>>>>>> > > >>>>>>> > > >>>>>> Ok, those are good points that I need to think about. > > >>>>>> > > >>>>>> Thanks for the help so far. I'd welcome any suggestions you have on > > >>>>>> how best to fix this. > > >>>>> For NFS, is it desirable to retry a redirtied inode after 30s, or > > >>>>> after a shorter 5s, or after 0.1~5s? Or the exact timing simply > > >>>>> doesn't matter? > > >>>>> > > >>>> I don't really consider NFS to be a special case here. It just happens > > >>>> to be where we saw the problem originally. Some of its characteristics > > >>>> might make it easier to hit this, but I'm not certain of that. > > >>> Now there are now two possible solutions: > > >>> - unconditionally update dirtied_when in redirty_tail(); > > >>> - keep dirtied_when and redirty inodes to a new dedicated queue. > > >>> The first one involves less code, the second one allows more flexible timing. > > >>> > > >>> NFS/XFS could be a good starting point for discussing the > > >>> requirements, so that we can reach a suitable solution. > > >>> > > >> It sounds like it, yes. I saw that you posted some patches in January > > >> (including your s_more_io_wait patch). I'll give those a closer look. > > >> Adding the new s_more_io_wait queue is interesting and might sidestep > > >> this problem nicely. > > >> > > > > > > Yes, I was looking at that bit of code but, so far, I think it won't be > > > called for the case we are trying to describe. > > > > I take that back. > > As Jeff pointed out I haven't seen these patches and can't seem to find > > them in my fsdevel list folder, Wu can you send me a copy please? > > > > Actually, I think you were right. We still have this check in > generic_sync_sb_inodes() even with Wu's January 2008 patches: > > /* Was this inode dirtied after sync_sb_inodes was called? */ > if (time_after(inode->dirtied_when, start)) > break; Yeah, ugly code. Jens' per-bdi flush daemons should eliminate it... > ...this check is the crux of the problem. We're assuming that the > dirtied_when value will never appear to be in the future. If we change > this check so that it's checking that dirtied_when is between "start" > and "now", then this problem basically goes away. Yeah that turns the problem into a temporary and tolerable one. > We'll probably also need to change the test in move_expired_inodes > too, unless Wu's changes go in. So the most simple (and complete) solution is still this one ;-) Thanks, Fengguang --- fs/fs-writeback.c | 14 +------------- 1 file changed, 1 insertion(+), 13 deletions(-) --- mm.orig/fs/fs-writeback.c +++ mm/fs/fs-writeback.c @@ -182,24 +182,12 @@ static int write_inode(struct inode *ino /* * Redirty an inode: set its when-it-was dirtied timestamp and move it to the * furthest end of its superblock's dirty-inode list. - * - * Before stamping the inode's ->dirtied_when, we check to see whether it is - * already the most-recently-dirtied inode on the s_dirty list. If that is - * the case then the inode must have been redirtied while it was being written - * out and we don't reset its dirtied_when. */ static void redirty_tail(struct inode *inode) { struct super_block *sb = inode->i_sb; - if (!list_empty(&sb->s_dirty)) { - struct inode *tail_inode; - - tail_inode = list_entry(sb->s_dirty.next, struct inode, i_list); - if (!time_after_eq(inode->dirtied_when, - tail_inode->dirtied_when)) - inode->dirtied_when = jiffies; - } + inode->dirtied_when = jiffies; list_move(&inode->i_list, &sb->s_dirty); }