linux-kernel - Re: [PATCH -mm v2 01/11] mm: track free size between VMAs in VMA rbtree

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Date:	Fri, 22 Jun 2012 11:41:33 -0400
From:	Rik van Riel <riel@...riel.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	akpm@...ux-foundation.org, aarcange@...hat.com, minchan@...il.com,
	kosaki.motohiro@...il.com, andi@...stfloor.org, hannes@...xchg.org,
	mel@....ul.ie, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 01/11] mm: track free size between VMAs in VMA
 rbtree

On 06/22/2012 10:37 AM, Peter Zijlstra wrote:
> On Fri, 2012-06-22 at 10:25 -0400, Rik van Riel wrote:
>> On 06/22/2012 10:13 AM, Peter Zijlstra wrote:
>>> On Fri, 2012-06-22 at 10:11 -0400, Rik van Riel wrote:
>>>>
>>>> I am still trying to wrap my brain around your alternative
>>>> search algorithm, not sure if/how it can be combined with
>>>> arbitrary address limits and alignment...
>>>
>>> for alignment we can do: len += align - 1;
>>
>> We could, but that might lead us to returning -ENOMEM
>> when we actually have memory available.
>>
>> When you consider architectures like HPPA, which use
>> a pretty large alignment, but align everything the same,
>> chances are pretty much every freed hole will have the
>> right alignment...
>
> Well, if you don't your gap heap is next to useless and you'll revert to
> simply walking all gaps until you find a suitable one.

I could see how that might potentially be a problem,
especially when we have a small allocation with large
alignment constraints, eg. HPPA cache alignment.

> I really worry about this search function of yours, its complexity is
> very non obvious.

Let me try implementing your algorithm with arbitrary
address constraints and alignment/colouring.

Basically, we need to remember if the allocation failed
due to bad alignment.  If it did, we add shm_align_mask
to the allocation length, and try a second search.

This should result in at worst two whole tree traversals
and one partial traversal. Less on sane architectures,
or for non-MAP_SHARED allocations.

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