Re: [PATCH] fs: Fix hang with BSD accounting on frozen filesystem

[Jan Kara <jack@suse.cz>]

On Thu 08-11-12 16:22:56, Dave Chinner wrote:
> On Wed, Nov 07, 2012 at 11:31:39PM +0100, Jan Kara wrote:
> > When BSD process accounting is enabled and logs information to a filesystem
> > which gets frozen, system easily becomes unusable because each attempt to
> > account process information blocks. Thus e.g. every task gets blocked in exit.
> > 
> > It seems better to drop accounting information (which can already happen when
> > filesystem is running out of space) instead of locking system up. This is
> > implemented using a special flag FMODE_NO_FREEZE_WAIT in file->f_mode of a
> > file to which accounting information is written.
> 
> I have no problems with making freeze waiting non-blocking, by why
> invent a new flag for what is essentially an O_NONBLOCK operation?
> 
> Indeed, if someone opens a file O_NONBLOCK, shouldn't if behave
> exactly the same on a frozen filesystem as this special
> FMODE_NO_FREEZE_WAIT flag?
  Originally I didn't want to cause user visible effects for other files
with the fix. But you are right that O_NONBLOCK means exactly what we need
and userspace could actually expect something like this to work. I'm
somewhat worried about broken applications which don't expect EAGAIN from
O_NONBLOCK write to a file (as traditionally O_NONBLOCK was no-op for
buffered file writes) but I guess it's worth a try. Thanks for idea, I'll
send an updated patch shortly.
 
> FWIW, nfsd could use this as well so that it doesn't block all the
> nfsd threads trying to write to a frozen filesystem but instead
> returns EJUKEBOX to the client to tell it ot wait for a while before
> trying the operation again...
  Yes, that would be another possible use of O_NONBLOCK.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR