linux-kernel - Re: [PATCH -mm 2/2] mm: do not reset mm->free_area

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Date:	Mon, 27 Feb 2012 11:12:42 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Mel Gorman <mel@....ul.ie>,
	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>, hughd@...gle.com
Subject: Re: [PATCH -mm 2/2] mm: do not reset mm->free_area_cache on every
 single munmap

On 02/23/2012 04:56 PM, Andrew Morton wrote:
> On Thu, 23 Feb 2012 15:00:34 -0500
> Rik van Riel<riel@...hat.com>  wrote:

>> The benefit is that things scale a lot better, and we remove about
>> 200 lines of code.
>
> We've been playing whack-a-mole with this search for many years.  What
> about developing a proper data structure with which to locate a
> suitable-sized hole in O(log(N)) time?

I have thought about this, and see a few different
possibilities:

1) Allocate a new (smaller) structure to keep track
    of free areas; this creates the possibility of
    munmap failing due to a memory allocation failure.
    It looks like it can already do that, but I do not
    like the idea of adding another failure path like
    it.

2) Use the vma_struct to keep track of free areas.
    Somewhat bloated, and may still not fix (1), because
    munmap can end up splitting a VMA.

I guess the free areas could be maintained in a prio tree,
sorted by both free area size and address, so we can fill
in the memory in the desired direction.

What I do not know is whether it will be worthwhile,
because the code I have now seems to behave well even
what is essentially a worst case scenario.

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