linux-kernel - Re: missing madvise functionality

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Date:	Wed, 4 Apr 2007 15:37:17 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Marko Macek <Marko.Macek@....net>
cc:	Ulrich Drepper <drepper@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Jakub Jelinek <jakub@...hat.com>
Subject: Re: missing madvise functionality

On Wed, 4 Apr 2007, Marko Macek wrote:
> Ulrich Drepper wrote:
> > A solution for this problem is a madvise() operation with the following
> > property:
> > 
> >   - the content of the address range can be discarded
> > 
> >   - if an access to a page in the range happens in the future it must
> >     succeed.  The old page content can be provided or a new, empty page
> >     can be provided
> 
> Doesn't this conflict with disabling overcommit?
> 
> If the page is guaranteed to be available, obviously it must count as
> being commited, so this is not equivalent to real freeing.

No, there's no conflict with disabled overcommit here: Committed_AS
accounting is done on the whole vma size (at mmap or brk time), no
matter how many pages may or may not be faulted in later.  Rather
like RLIMIT_AS.  The proposed madvise operation won't affect it.

(But I take Ulrich's "must succeed" with one pinch of salt:
Out-Of-Memory killing remains a possibility, of course.)

Hugh
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