On Mon, 2025-03-24 at 12:38 +0100, Jan Kara wrote: > On Fri 21-03-25 13:00:24, James Bottomley via Lsf-pc wrote: > > On Fri, 2025-03-21 at 08:34 -0400, James Bottomley wrote: > > [...] > > > Let me digest all that and see if we have more hope this time > > > around. > > > > OK, I think I've gone over it all. The biggest problem with > > resurrecting the patch was bugs in ext3, which isn't a problem now. > > Most of the suspend system has been rearchitected to separate > > suspending user space processes from kernel ones. The sync it > > currently does occurs before even user processes are frozen. I > > think > > (as most of the original proposals did) that we just do freeze all > > supers (using the reverse list) after user processes are frozen but > > just before kernel threads are (this shouldn't perturb the image > > allocation in hibernate, which was another source of bugs in xfs). > > So as far as my memory serves the fundamental problem with this > approach was FUSE - once userspace is frozen, you cannot write to > FUSE filesystems so filesystem freezing of FUSE would block if > userspace is already suspended. You may even have a setup like: > > bdev <- fs <- FUSE filesystem <- loopback file <- loop device <- > another fs > > So you really have to be careful to freeze this stack without causing > deadlocks. Ah, so that explains why the sys_sync() sits in suspend resume *before* freezing userspace ... that always appeared odd to me. > So you need to be freezing userspace after filesystems are > frozen but then you have to deal with the fact that parts of your > userspace will be blocked in the kernel (trying to do some write) > waiting for the filesystem to thaw. But it might be tractable these > days since I have a vague recollection that system suspend is now > able to gracefully handle even tasks in uninterruptible sleep. There is another thing I thought about: we don't actually have to freeze across the sleep. It might be possible simply to invoke freeze/thaw where sys_sync() is now done to get a better on stable storage image? That should have fewer deadlock issues. > > There's a final wrinkle in that if I plumb efivarfs into all this, > > it needs to know whether it was a hibernate or suspend, but I can > > add that as an extra freeze_holder flag. > > > > This looked like such a tiny can of worms when I opened it; now it > > seems to be a lot bigger on the inside than it was on the outside, > > sigh. > > Never underestimate the amount of worms in a can ;) Tell me about it ... Regards, James
