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Date: Fri, 19 Nov 2010 10:06:41 -0500
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback
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To: Theodore Tso <tytso@google.com>
Cc: Michel Lespinasse <walken@google.com>, Hugh Dickins <hughd@google.com>, Christoph Hellwig <hch@infradead.org>, Dave Chinner <david@fromorbit.com>, Peter Zijlstra <peterz@infradead.org>, Nick Piggin <npiggin@kernel.dk>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>, Rik van Riel <riel@redhat.com>, Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>, Michael Rubin <mrubin@google.com>, Suleiman Souhlal <suleiman@google.com>
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On Fri, Nov 19, 2010 at 08:42:05AM -0500, Theodore Tso wrote:
> My vote would be against. ? If you if you mmap a sparse file and then
> try writing to it willy-nilly, bad things will happen. ?This is true without
> a mlock(). ? Where is it written that mlock() has anything to do with
> improving this situation?

Exactly.  Allocating space has been a side-effect on a handfull
filesystem for about 20 kernel releases.

> If userspace wants to call fallocate() before it calls mlock(), it should
> do that. ?And in fact, in most cases, userspace should probably be
> encouraged to do that. ? But having mlock() call fallocate() and
> then return ENOSPC if there's no room?  Isn't it confusing that mlock()
> call ENOSPC?  Doesn't that give you cognitive dissonance?  It should
> because fundamentally mlock() has nothing to do with block allocation!!
> Read the API spec!

Indeed.  There is no need to make mlock + flag a parallel-API to
fallocate.

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