linux-kernel - Re: missing madvise functionality

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Date:	Wed, 4 Apr 2007 16:25:24 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Rik van Riel <riel@...hat.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jakub Jelinek <jakub@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: missing madvise functionality

On Wed, 4 Apr 2007, Rik van Riel wrote:
> Hugh Dickins wrote:
> 
> > (I didn't understand how Rik would achieve his point 5, _no_ lock
> > contention while repeatedly re-marking these pages, but never mind.)
> 
> The CPU marks them accessed&dirty when they are reused.
> 
> The VM only moves the reused pages back to the active list
> on memory pressure.  This means that when the system is
> not under memory pressure, the same page can simply stay
> PG_lazyfree for multiple malloc/free rounds.

Sure, there's no need for repetitious locking at the LRU end of it;
but you said "if the system has lots of free memory, pages can go
through multiple free/malloc cycles while sitting on the dontneed
list, very lazily with no lock contention".  I took that to mean,
with userspace repeatedly madvising on the ranges they fall in,
which will involve mmap_sem and ptl each time - just in order
to check that no LRU movement is required each time.

(Of course, there's also the problem that we don't leave our
systems with lots of free memory: some LRU balancing decisions.)

Hugh
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