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

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Message-ID: <20080708232031.GE18195@elf.ucw.cz>
Date:	Wed, 9 Jul 2008 01:20:31 +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

On Wed 2008-07-09 09:10:27, Dave Chinner wrote:
> On Mon, Jul 07, 2008 at 01:07:31PM +0200, Pavel Machek wrote:
> > 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?

> 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? And dirty data mean
memory pressure, right? 

> d) if dirtying another unfrozen filesystem swaps out your
                 ~~~~~~~
>    application so it can't run, then there's a major VM bug.
>    Regardless, until the app completes it is relying on the
>    filesystem being frozen, so it better remain frozen....

Agreed. With emphasis on "another".

> > I though that was why the timeout was there...
> 
> Not that I know of. 

Ok, lets see how you deal with mmap.
								Pavel

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