On Tuesday, August 23, 2011, Dave Chinner wrote: > On Mon, Aug 22, 2011 at 03:00:45PM +0200, Pavel Machek wrote: > > Hi! > > > > > > What's exactly the problem? Memory preallocation enters direct reclaim > > > > and that deadlocks in the filesystem? > > > > > > Well, that's one possible manifestation. The problem is that the > > > current hibernate code still assumes that sys_sync() results in an > > > idle filesystem that will not change after the call if nothing is > > > dirty. > > > > > > The result is that when the large memory allocation occurs for the > > > hibernate image (after the sys_sync() call) then the shrink_slab() > > > tends to be called. The XFS shrinkers are capable of dirtying inodes > > > and the backing buffers of inodes that are in the reclaimable state. > > > But those buffers cannot be flushed to disk because hibernate has > > > already frozen the xfsbufd threads, so the shrinker doing inode > > > reclaim hangs up on locks waiting for the buffers to be written. > > > This either leads to deadlock or hibernate image allocation failure. > > > > > > Far worse, IMO, is the case where is -doesn't- deadlock, because the > > > filesystem state can still changing after the allocation has > > > finished due to async metadata IO completions. That has the > > > potential to cause filesystem corruption as after resume the on-disk > > > state may not match what is written from memory to the hibernate > > > image. > > > > > > The problem really isn't XFS specific, nor is it new - the fact is > > > that any filesystem that has registered a shrinker or can do async > > > work in the background post-sync is vulnerable to this problem. It's > > > > Should we avoid calling shrinkers while hibernating? > > If you like getting random OOM problems when hibernating, then go > for it. Besides, shrinkers are used for more than just filesystems, > so you might find you screw entire classes of users by doing this > (eg everyone using intel graphics and 3D). > > > Or put BUG_ON()s into filesystem shrinkers so that this can not > > happen? > > Definitely not. If your concern is filesystem shrinkers and you want > a large hammer to hit the problem with then do your hibernate > image allocation wih GFP_NOFS and the filesystem shrinkers will > abort without doing anything. I think we can do that, actually. Thanks, Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
