On Wednesday, July 27, 2011, Dave Chinner wrote: > On Tue, Jul 26, 2011 at 10:28:11PM +0200, Rafael J. Wysocki wrote: > > On Wednesday, July 13, 2011, Dave Chinner wrote: > > > On Tue, Jul 12, 2011 at 06:05:01PM +0200, Christoph wrote: > ..... > > > > SysRq : Show Blocked State > > > > > > > > pm-hibernate D 0000000000000000 0 3638 3637 0x00000000 > > > > ffff8800017bf918 0000000000000082 ffff8800017be010 ffff880000000000 > > > > ffff8800017be010 ffff88000b8a6170 0000000000013900 ffff8800017bffd8 > > > > ffff8800017bffd8 0000000000013900 ffffffff8148b020 ffff88000b8a6170 > > > > Call Trace: > > > > [<ffffffff81344ce2>] schedule_timeout+0x22/0xbb > > > > [<ffffffff81344b64>] wait_for_common+0xcb/0x148 > > > > [<ffffffff810408ea>] ? try_to_wake_up+0x18c/0x18c > > > > [<ffffffff81345527>] ? down_write+0x2d/0x31 > > > > [<ffffffff81344c7b>] wait_for_completion+0x18/0x1a > > > > [<ffffffffa02374da>] xfs_reclaim_inode+0x74/0x258 [xfs] > > > > [<ffffffffa0237853>] xfs_reclaim_inodes_ag+0x195/0x264 [xfs] > > > > [<ffffffffa0237974>] xfs_reclaim_inode_shrink+0x52/0x90 [xfs] > > > > [<ffffffff810c4e21>] shrink_slab+0xdb/0x151 > > > > [<ffffffff810c625a>] do_try_to_free_pages+0x204/0x39a > > > > [<ffffffff8134ce4e>] ? apic_timer_interrupt+0xe/0x20 > > > > [<ffffffff810c647f>] shrink_all_memory+0x8f/0xa8 > > > > [<ffffffff810cc41a>] ? next_online_pgdat+0x20/0x41 > > > > [<ffffffff8107937d>] hibernate_preallocate_memory+0x1c4/0x30f > > > > [<ffffffff811a8fa2>] ? kobject_put+0x47/0x4b > > > > [<ffffffff81077eb2>] hibernation_snapshot+0x45/0x281 > > > > [<ffffffff810781bf>] hibernate+0xd1/0x1b8 > > > > [<ffffffff81076c58>] state_store+0x57/0xce > > > > [<ffffffff811a8d0b>] kobj_attr_store+0x17/0x19 > > > > [<ffffffff81152bda>] sysfs_write_file+0xfc/0x138 > > > > [<ffffffff810fca74>] vfs_write+0xa9/0x105 > > > > [<ffffffff810fcb89>] sys_write+0x45/0x6c > > > > [<ffffffff8134c492>] system_call_fastpath+0x16/0x1b > > > > > > It's waiting for IO completion, and holding an AG scan lock. > > > > > > And IO completion requires a workqueue to run. Just FYI, this > > > process of inode reclaim can dirty the filesystem, long after > > > hibernate have assumed that it is clean due to the sys_sync() call > > > you do after freezing the processes. I pointed out this flaw in > > > using sync to write dirty data prior to hibernate a couple of years > > > ago. > > > > However, attempts to remove the sys_sync() from the hibernate code > > were objected to by some developers, since they believe it will increase > > the probability of data loss in case of a failing hibernation in general. > > I'm not suggesting it gets removed, I'm suggesting it gets replaced > because it doesn't give the guarantees that you want or need. > > > > Anyway, it's a good thing that XFS doesn't use freezable work > > > queues, otherwise it would hang on every hibernate. Perhaps I should > > > do that to force hibernate to do things properly in filesystems > > > land. > > > > Well, I'd say it's a very well known fact that filesystems are not > > handled in any special way during hibernation, which is not a good > > thing. Nevertheless, I've never seen anyone from the filesystems land > > pay any kind of attention to this issue. > > I beg to differ. We went through this exact clas of bugs with swsusp > back in 2006: > > https://lkml.org/lkml/2006/11/12/144 You're right, sorry. We discussed this 5 years ago, but the context was a bit different (the hibernation code was using a different mechanism for freeing memory). > And this patch: > > https://lkml.org/lkml/2006/11/1/155 > > ([PATCH -mm] swsusp: Freeze filesystems during suspend) > > "This is needed by swsusp, because some filesystems (eg. XFS) use > work queues and worker_threads run with PF_NOFREEZE set, so they can > cause some writes to be performed after the suspend image has been > created which may corrupt the filesystem. The additional benefit of > it is that if the resume fails, the filesystems will be in a > consistent state and there won't be any journal replays needed." > > -- > > And the patch essentially does: > > - sys_sync(); > + freeze_filesystems(); Well, if you still think the patch does the right thing, I can rebase it on top of the current freezer code and resubmit. > But, Pavel didn't like freezing filesystems to quiesce them > correctly, so the sys_sync() and all it's problems have remained > until this day, where we still have users tripping over the same > "filesystem not idle" problems. The problem seems to be quite specific to XFS, though. The Pavel's objection, if I remember it correctly, was that some (or the majority of?) filesystems didn't implement the freezing operation, so they would be more vulnerable to data loss in case of a failing hibernation after this change. However, that's better than actively causing pain to XFS users. > > [....] > > > > IOWs, what hibernate does is: > > > > > > freeze_processes() > > > sys_sync() > > > allocate a large amount of memory > > > > > > Freezing the processes causes parts of filesystems to be put in the > > > fridge, which means there is no guarantee that sys_sync() actually > > > does what it is supposed to. As it is, sys_sync() really only > > > guarantees file data is clean in memory - metadata does not need to > > > be clean as long s it has been journalled and the journal is safe on > > > disk. > > > > > > Further, allocating memory can cause memory reclaim to enter the > > > filesystem and try to free memory held by the filesystem. In XFS (at > > > least) this can cause the filesystem to issue tranactions and > > > metadata IO to clean the dirty metadata to enable it to be > > > reclaimed. So hibernate is effectively guaranteed to dirty the > > > filesystem after it has frozen all the worker threads the filesystem > > > might rely on. > > > > > > Also, by this point kswapd has already been frozen, so hibernate is > > > relying totally on direct memory reclaim to free up the memory it > > > requires. I'm not sure that's a good idea. > > > > > > IOWs, hibernate is still broken by design - and broken in exactly > > > the way that was pointed out a couple of years ago by myself and > > > others in the filesystem world: sys_sync() does not quiesce or > > > guarantee a clean filesystem in memory after it completes. > > > > > > There is a solution to this, and it already exists - it's called > > > freezing the filesystem. Effectively hibernate needs to allocate > > > memory before it freezes kernel/filesystem worker threads: > > > > > > freeze_userspace_processes() > > > > > > // just to clean the page cache quickly > > > sys_sync() > > > > > > // optionally to free page/inode/dentry caches: > > > iterate_supers(drop_pagecache_sb, NULL); > > > drop_slab() > > > > > > allocate a large amount of memory > > > > > > // Now quiesce the filesystems and clean remaining metadata > > > iterate_supers(freeze_super, NULL); > > > > > > freeze_remaining_processes() > > > > > > This guarantees that filesystems are still working when memory > > > reclaim comes along to free memory for the hibernate image, and that > > > once it is allocated that filesystems will not be changed until > > > thawed on the hibernate wakeup. > > > > > > So, like I said a couple of years ago: fix hibernate to quiesce > > > filesystems properly, and the hibernate will be much more reliable > > > and robust and less likely to break randomly in the future. > > > > Why don't you simply submit a patch to do that? > > a) I don't know how to test suspend/hibernate > b) I don't have any hardware I can test it on. > c) I don't scale to solving every problem Linux has > d) you guys need to decide how you're going to fix this because the > problem has already been solved once before and it didn't get merged > because nobody in the swsusp/hibernate world could agree on anything > at the time. OK, I'm not a filesystem expert in turn. As I said, I can revive the Nigel's patch if you think it's better than the code we have, but I'm afraid that's all I can do without any help from the filesystems people. Thanks, Rafael _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
