On Monday, 26 of November 2007, Jeremy Fitzhardinge wrote:
> Rafael J. Wysocki wrote:
> > On Thursday, 22 of November 2007, Jeremy Fitzhardinge wrote:
> >> It seems that a process blocked in a write to an xfs filesystem due to
> >> xfs_freeze cannot be frozen by the freezer.
> > The freezer doesn't handle tasks in TASK_UNINTERRUPTIBLE and I don't know
> > how
> > to make it handle them without at least partially defeating its purpose.
> Well, I guess the question is whether an xfs-frozen writer really needs
> to be UNINTERRUPTIBLE from the freezer's perspective (clearly it does
> from usermode's perspective - filesystem writes just don't return EINTR).
> From a quick poke around, it looks to me like freezing is actually
> implemented in the VFS layer rather than in XFS itself: is that right?
I don't know the details.
> Could vfs_check_frozen() be changed to something that is freezer-compatible?
That seems doable in principle. I'll have a closer look at it.
> >> I see this if I suspend my laptop while doing something xfs-filesystem
> >> intensive, like a kernel build. My suspend scripts freeze the XFS
> >> filesystem (as Dave said I should), which presumably blocks some writer,
> >> and then the freezer times out and fails to complete.
> >> Here's part of the process dump the freezer does when it times out:
> >> cc1 D 00000000 0 18138 18137
> >> dd5f1e24 00200082 00000002 00000000 ecdeeb00 ecdeec64 c200f280
> >> 00000001
> >> 009c09a0 dd5f1e0c dd5f1e0c 0000000f 00000000 00000000 00000000
> >> dd5f1e74
> >> c7beb480 dd5f1e88 dd5f1ea8 c0228d97 e8889540 dd5f1e38 c015b75d
> >> dd5f1e44
> >> Call Trace:
> >> [<c0228d97>] xfs_write+0xf4/0x6d9
> >> [<c0226038>] xfs_file_aio_write+0x53/0x5b
> >> [<c0171c15>] do_sync_write+0xae/0xec
> >> [<c0172343>] vfs_write+0xa4/0x120
> >> [<c01728d7>] sys_write+0x3b/0x60
> >> [<c0106fae>] sysenter_past_esp+0x6b/0xa1
> >> =======================
> >> I haven't looked at how to fix this yet. I only just worked out why I
> >> was getting suspend failures.
> > Well, you can add freezer_do_not_count()/freezer_count() annotations to
> > xfs_write() (and whatever else is blocked as a result of the XFS being
> > frozen).
> What would be the implications of that? Would that just prevent
> freezing while there's something blocked there?
The freezer will not wait for this particular task. Still, the task will have
TIF_FREEZE set, so it will freeze as soon as freezer_count() is reached,
unless the thawing of tasks is carried out first.
If used in the right place, it's reasonably safe, but we need to know what
the right place is. [That's how we handle vfork(), BTW.]
> > Generally, that would be risky without the freezing of XFS, however,
> > because it
> > might leak us filesystem data to a storage device after creating a
> > hibernation
> > image which would result in the filesystem corruption after the resume.
> > Still, if you only suspend to RAM, that should be safe.
> I specifically added it because I was getting data loss due to crashes
> during suspend/resume problems. It's been pretty stable lately, but I
> may as well remove the xfs_freeze from my suspend scripts if this is the
Not exactly. :-)
> I think the broader issue is that there's no reason in principle why
> something blocked due to xfs-freezing (or vfs freezing) should prevent
> the freezer from completing.
Agreed, but the only way to tell the freezer "don't wait for this task", if the
task in question is in TASK_UNINTERRUPTIBLE, is to annotate it.