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Re: [PATCH 3/3] Add timeout feature

To: arjan@xxxxxxxxxxxxx
Subject: Re: [PATCH 3/3] Add timeout feature
From: Miklos Szeredi <miklos@xxxxxxxxxx>
Date: Wed, 09 Jul 2008 09:08:07 +0200
Cc: miklos@xxxxxxxxxx, miklos@xxxxxxxxxx, hch@xxxxxxxxxxxxx, tytso@xxxxxxx, pavel@xxxxxxx, t-sato@xxxxxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, viro@xxxxxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, dm-devel@xxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, axboe@xxxxxxxxx, mtk.manpages@xxxxxxxxxxxxxx
In-reply-to: <20080708235502.1c52a586@xxxxxxxxxxxxx> (message from Arjan van de Ven on Tue, 8 Jul 2008 23:55:02 -0700)
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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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