linux-kernel - Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback

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Date:	Fri, 19 Nov 2010 10:06:41 -0500
From:	Christoph Hellwig <hch@...radead.org>
To:	Theodore Tso <tytso@...gle.com>
Cc:	Michel Lespinasse <walken@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <david@...morbit.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Piggin <npiggin@...nel.dk>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Kosaki Motohiro <kosaki.motohiro@...fujitsu.com>,
	Michael Rubin <mrubin@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback

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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