Re: [PATCH 2/2] fs: print a message when freezing/unfreezing filesystems

Mateusz Guzik
2014-05-15T09:42:49Z

On Thu, May 15, 2014 at 07:54:57AM +1000, Dave Chinner wrote:
> On Tue, May 13, 2014 at 08:31:02PM +0200, Mateusz Guzik wrote:
> > This helps hang troubleshooting efforts when only dmesg is available.
>
> I really don't think that spamming dmesg every time a filesystem is
> frozen or thawed is a good idea. This happens a *lot* when systems
> are using snapshots, and for the most part nobody cares about
> freeze/thaw cycles because they almost always work just fine.
>

I agree it may get noisy.

> I'd think that /proc/self/mounts would be a much better place to
> indicate that the fs is frozen. After all, that's where we tell
> people whether the filesystem is ro or rw, and frozen is just
> a temporary, non-invasive ro state...
>

Except you can't inspect /proc/self/mounts when the only thing you got
is dmesg, so this does not really help my case.

That said, I'll try to come up with a different solution.

Poorly reported side-effects of frozen I/O are only a part of the real
problem which is hung task detector being able to typically report
backtraces of "victims" only. I came up with printks becuase these are
a cheap way and would help us out in a lot of cases.

So general idea is to support providing callbacks when setting tasks to
TASK_UNINTERRUPTIBLE which could either tell the user what's up directly
or would perform some heuristics. I'll post this in a separate thread
later, maybe with PoC.

Thanks,
--
Mateusz Guzik