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

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Message-Id: <E1KGTmZ-0006oz-Qi@pomaz-ex.szeredi.hu>
Date:	Wed, 09 Jul 2008 09:08:07 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	arjan@...radead.org
CC:	miklos@...redi.hu, miklos@...redi.hu, hch@...radead.org,
	tytso@....edu, pavel@...e.cz, t-sato@...jp.nec.com,
	akpm@...ux-foundation.org, viro@...IV.linux.org.uk,
	linux-ext4@...r.kernel.org, xfs@....sgi.com, dm-devel@...hat.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	axboe@...nel.dk, mtk.manpages@...glemail.com
Subject: Re: [PATCH 3/3] Add timeout feature

On Tue, 8 Jul 2008, Arjan van de Ven wrote:
> I tihnk the idea there is
> 
> freeze . do the snapshot op . unfreeze . make backup of snapshot

Ah, so then my proposal would become

  run_frozen mountpoint do-snapshot
  do-backup
  release-snapshot

and if they are afraid of deadlocks they can just implement the
timeout in userspace:

  run_frozen -t timeout mountpoint do-snapshot

'run_frozen' can be a trivial 30 line app, that can be guaranteed not
to deadlock.

> one can argue about the need of doing the first 3 steps via a userland
> loop; it sure sounds like one needs to be really careful to not do any
> writes to the fs from the app that does snapshots (and that includes
> doing any syscalls in the kernel that allocate memory.. just because
> that already could cause unrelated data to be written from inside the
> app. Not fun.)

Userland always has to be careful when messing with raw devices.  That
alone is not a reason to put the snapshotting facility in kernel IMO.

Miklos
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