linux-kernel - Re: [PATCH] lazy freeing of memory through MADV

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Date:	Sat, 21 Apr 2007 08:37:03 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Ulrich Drepper <drepper@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] lazy freeing of memory through MADV_FREE 2/2

On Fri, 20 Apr 2007, Ulrich Drepper wrote:
> 
> Just for reference: the MADV_CURRENT behavior is to throw away data in
> the range.

Not exactly.  The Linux MADV_DONTNEED never throws away data from a
PROT_WRITE,MAP_SHARED mapping (or shm) - it propagates the dirty bit,
the page will eventually get written out to file, and can be retrieved
later by subsequent access.  But the Linux MADV_DONTNEED does throw away
data from a PROT_WRITE,MAP_PRIVATE mapping (or brk or stack) - those
changes are discarded, and a subsequent access will revert to zeroes
or the underlying mapped file.  Been like that since before 2.4.0.

> The POSIX_MADV_DONTNEED behavior is to never lose data.
> I.e., file backed data is written back, anon data is at most swapped
> out.
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