linux-kernel - Re: vma

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Message-ID: <Pine.LNX.4.64.0908122143020.20776@sister.anvils>
Date:	Wed, 12 Aug 2009 21:53:25 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Bill Speirs <bill.speirs@...il.com>
cc:	Nick Piggin <npiggin@...e.de>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: vma_merge issue

On Wed, 12 Aug 2009, Hugh Dickins wrote:
> 
> Don't use anonymous memory, have a 1GB sparse file to back this,
> and mmap it MAP_SHARED, then you won't get charged for RAM+swap.

A "refinement" to that suggestion is to put the file on tmpfs:
you will then get charged for RAM+swap as you use it, but you can
use madvise MADV_REMOVE to unmap pages, punching holes in the file,
freeing up those charges.  A little baroque, but I think it does
amount to a way of doing exactly what you wanted in the first place.

(Note: we do insist on PROT_WRITE access at the time of MADV_REMOVE:
I've even a feeling it was me who insisted on that.)

Hugh
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