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Re: [PATCH 3/3] Add timeout feature

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>, Takashi Sato <t-sato@xxxxxxxxxxxxx>, akpm@xxxxxxxxxxxxxxxxxxxx, viro@xxxxxxxxxxxxxxxxxx, "linux-ext4@xxxxxxxxxxxxxxx" <linux-ext4@xxxxxxxxxxxxxxx>, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>, "dm-devel@xxxxxxxxxx" <dm-devel@xxxxxxxxxx>, "linux-fsdevel@xxxxxxxxxxxxxxx" <linux-fsdevel@xxxxxxxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, axboe@xxxxxxxxx, mtk.manpages@xxxxxxxxxxxxxx
Subject: Re: [PATCH 3/3] Add timeout feature
From: Pavel Machek <pavel@xxxxxxx>
Date: Wed, 9 Jul 2008 22:44:13 +0200
In-reply-to: <20080709005254.GQ11558@disturbed>
References: <20080630212450t-sato@xxxxxxxxxxxxxxx> <20080701081026.GB16691@xxxxxxxxxxxxx> <20080707110730.GG5643@xxxxxx> <20080708231026.GP11558@disturbed> <20080708232031.GE18195@xxxxxxxxxx> <20080709005254.GQ11558@disturbed>
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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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