Good morning! On Thursday 05 July 2007 23:59:57 Rafael J. Wysocki wrote: > On Thursday, 5 July 2007 15:36, Nigel Cunningham wrote: > > On Thursday 05 July 2007 23:35:45 Rafael J. Wysocki wrote: > > > On Thursday, 5 July 2007 14:38, Nigel Cunningham wrote: > > > > On Thursday 05 July 2007 22:25:06 Rafael J. Wysocki wrote: > > > > > On Thursday, 5 July 2007 01:45, Pavel Machek wrote: > > > > > > On Tue 2007-07-03 21:32:20, Oliver Neukum wrote: > > > > > > > Am Dienstag, 3. Juli 2007 schrieb Miklos Szeredi: > > > > > > > > > And a further question. The freezer is not atomic. What do you > > do > > > > > > > > > if a task not yet frozen calls sys_sync(), but fuse is already > > > > frozen? > > > > > > > > > > > > > > > > What do you do if a task not yet frozen writes to a pipe, on the > > other > > > > > > > > end of which is a task already frozen? > > > > > > > > > > > > There's some difference between uninterruptible and interruptible > > > > > > sleep I'd say. > > > > > > > > > > > > > > It doesn't matter. The only thing that should matter during > > suspend > > > > > > > > (not hibernate) is saving the state of devices to ram, and putting > > the > > > > > > > > devices to sleep. > > > > > > > > > > > > > > Well, but you did remove sys_sync() from the freezer, which is > > > > > > > and must be called in the hibernate path. > > > > > > > > > > > > Not "must". In fact, hibernation should be safe without sys_sync(). It > > > > > > is just user un-friendly. > > > > > > > > > > In fact, I'd like to remove the sys_sync() from the freezer entirely, > > > > because > > > > > it just doesn't belong in there. > > > > > > > > > > The only advantege of having sys_sync() in freeze_processes() is that we > > > > > have a chance to write out everything when applications cannot produce > > more > > > > > data to write, but there are filesystems which don't do that anyway (eg. > > > > XFS), > > > > > so generally there's no reason to bother. > > > > > > > > Shouldn't XFS - and fuse - be considered to be broken? Sync should sync > > data > > > > and if XFS isn't doing that, it's wrong. > > > > > > > > In the case of fuse, we should have a mechanism by which fuse processes > > can be > > > > made to sync if they do have any pending I/O, and by which they can be > > frozen > > > > later than other userspace processes. > > > > > > > > I'd like to see the sync stay, because it improves reliability and data > > > > integrity in the fail-to-resume case. Calling scripts would probably > > invoke > > > > sync themselves if they don't already, but that's racy. As it is at the > > > > moment, we know userspace is stopped, so syncing isn't racy. > > > > > > I'd like to move the sync out of the freezer, but to call it from the > > > suspend/hibernation code, so that we do > > > > > > sys_sync(); > > > error = freeze_processes(); > > > > Yeah, I understand that. The problem then is that you're racing against > > userspace. That's not usually a problem, but that doesn't mean it's never a > > problem. Try running the stress suite while testing hibernating and you'll > > see what I mean. If something is submitting lots of I/O when you try to > > suspend, your sync call will race against that process if it's not yet > > frozen, and its continued activity will make your sync pointless (there'll be > > more unsynced data when you sys_sync call finishes). Stopping userspace > > before syncing removes that race. > > Yes, that will make the suspend/hibernation less reliable in case the resume > fails (some data, written after the sync, may be lost). However, the sync done > from within the freezer doesn't guarantee that there are no data lost anyway, > so we don't lose much by not doing it. > > Now, there's a question how much data may be lost, potentially, if we do the > sync before the freezer and I don't think that's a lot. You're missing the point. I'm arguing that a sync from within the freezer should guarantee that there is no data loss. As I said about, XFS should be fixed to properly sync its data, and something should be done about fuse filesystems too. Regards, Nigel -- See http://www.tuxonice.net for Howtos, FAQs, mailing lists, wiki and bugzilla info.
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