linux-ext4 - Re: [PATCH 3/3] Add timeout feature

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Date:	Wed, 9 Jul 2008 22:44:13 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Christoph Hellwig <hch@...radead.org>,
	Takashi Sato <t-sato@...jp.nec.com>, akpm@...ux-foundation.org,
	viro@...IV.linux.org.uk,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	axboe@...nel.dk, mtk.manpages@...glemail.com
Subject: Re: [PATCH 3/3] Add timeout feature

Hi!

> > > > > I still disagree with this whole patch.  There is not reason to let
> > > > > the freeze request timeout - an auto-unfreezing will only confuse the
> > > > > hell out of the caller.  The only reason where the current XFS freeze
> > > > > call can hang and this would be theoretically useful is when the
> > > > 
> > > > What happens when someone dirties so much data that vm swaps out
> > > > whatever process that frozen the filesystem?
> > > 
> > > a) you can't dirty a frozen filesystem - by definition a frozen
> > >    filesystem is a *clean filesystem* and *cannot be dirtied*.
> > 
> > Can you stop me?
> > 
> > mmap("/some/huge_file", MAP_SHARED);
> > 
> > then write to memory mapping?
> 
> Sure - we can put a hook in ->page_mkwrite() to prevent it.  We
> don't right now because nobody in the real world really cares if one
> half of a concurrent user data change is in the old snapshot or the
> new one......
> 
> > > b) Swap doesn't write through the filesystem
> > > c) you can still read from a frozen filesystem to page your
> > >    executable?? in.
> > 
> > atime modification should mean dirty data, right?
> 
> Metadata, not data. If that's really a problem (and it never has
> been for XFS because we always allow in memory changes to atime)
> then touch_atime could be easily changed to avoid this...
> 
> > And dirty data mean
> > memory pressure, right? 
> 
> If you walk enough inodes while the filesystem is frozen, it
> theoretically could happen.  Typically a filesystem is only for a
> few seconds at a time so in the real world this has never, ever been
> a problem.

So we have freezing interface that does not really freeze, and
that can break the system when filesystem is frozen for too long...
:-(.

Maybe you could use process freezer -- cgroup people are adding
userspace interface to that -- to solve those... but that would mean
stopping everyone but thread doing freezing...

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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