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. -- 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
