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Date: Mon, 29 Dec 2003 01:30:40 -0800
From: Andrew Morton <akpm@osdl.org>
To: Paul Jakma <paul@clubi.ie>
Cc: linux-kernel@vger.kernel.org
Subject: Re: chmod of active swap file blocks
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Paul Jakma <paul@clubi.ie> wrote:
>
> Hi,
> 
> Trying to chmod a file being used for swap causes chmod() to block,
> with permissions change /not/ having taken effect, until the swap
> file is swapoff'd, at which point chmod() carries on and chmod (the
> command) finishes.

The kernel holds the swapfile's i_sem while it is in use.  This is to
prevent the filesystem destruction which would result if some silly person
were to truncate a swapfile while it was in active use.

It is not a particularly important safety feature ("don't do that") and it
can be taken out if it is causing serious side-effects.

Is chmod of an in-use swapfile an important thing to be able to do?