linux-kernel - Re: missing madvise functionality

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Message-ID: <4612C401.3010507@redhat.com>
Date:	Tue, 03 Apr 2007 17:15:45 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Ulrich Drepper <drepper@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Jakub Jelinek <jakub@...hat.com>, linux-mm@...ck.org,
	Hugh Dickins <hugh@...itas.com>
Subject: Re: missing madvise functionality

Eric Dumazet wrote:
> Rik van Riel a écrit :
>> Andrew Morton wrote:
>>
>>> Oh.  I was assuming that we'd want to unmap these pages from 
>>> pagetables and
>>> mark then super-easily-reclaimable.  So a later touch would incur a 
>>> minor
>>> fault.
>>>
>>> But you think that we should leave them mapped into pagetables so no 
>>> such
>>> fault occurs.
>>
>>> Leaving the pages mapped into pagetables means that they are 
>>> considerably
>>> less likely to be reclaimed.
>>
>> If we move the pages to a place where they are very likely to be
>> reclaimed quickly (end of the inactive list, or a separate
>> reclaim list) and clear the dirty and referenced lists, we can
>> both reclaim the page easily *and* avoid the page fault penalty.
>>
> 
> There is one possible speedup :
> 
> - If an user app does a madvise(MADV_DONTNEED), we can assume the pages 
> can later be bring back without need to zero them. The application 
> doesnt care.

... however, the application that previously used that page might
care a lot!

> mmap()/brk() must give fresh NULL pages, but maybe 
> madvise(MADV_DONTNEED) can relax this requirement (if the pages were 
> reclaimed, then a page fault could bring a new page with random content)

If we bring in a new page, it has to be zeroed for security
reasons.

You don't want somebody else's process to get a page with
your password in it.

-- 
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