On Wed, 25 Mar 2009 22:38:47 +0900 Ian Kent <raven@xxxxxxxxxx> wrote: > Ian Kent wrote: > > Jeff Layton wrote: > >> On Wed, 25 Mar 2009 20:17:43 +0800 > >> Wu Fengguang <fengguang.wu@xxxxxxxxx> 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 <fengguang.wu@xxxxxxxxx> 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; ...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. We'll probably also need to change the test in move_expired_inodes too, unless Wu's changes go in. -- Jeff Layton <jlayton@xxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
