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

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Date:	Tue, 26 Jun 2012 11:49:48 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Rik van Riel <riel@...riel.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, danielfsantos@....net
Subject: Re: [PATCH -mm v2 01/11] mm: track free size between VMAs in VMA
 rbtree

On 06/26/2012 09:45 AM, Peter Zijlstra wrote:
> On Tue, 2012-06-26 at 09:05 -0400, Rik van Riel wrote:
>> On 06/26/2012 04:31 AM, Peter Zijlstra wrote:
>>
>>> If you look at your patch 1, __vma_unlink has an adjust_free_gap() right
>>> next to the rb_augment_erase(), vma_adjust() has 3 adjust_free_gap()
>>> calls right next to each other.
>>>
>>> All these will do an entire path walk back to the root. I would think we
>>> could save quite a bit of updating by not having them all walk back to
>>> the root. No point in re-computing the top levels if you know the next
>>> update will change them again anyway.
>>
>> The problem is, unless we look at the augmented data at
>> rotate time, we do not know when it is safe to stop
>> iterating up the tree.
>
> argh,.. you're using adjust_vma_gap() for insertions instead of
> rb_augment_insert().
>
> I was going on the premise that you're doing updates for augmented data
> without modifying the tree structure and that doing insert/delete will
> keep the stuff up-to-date.
>
> So now I'm not sure why you do if (insert) adjust_free_gap(insert),
> since __insert_vm_struct(mm, insert) ->  __vma_link() ->  __vma_link_rb()
> already does an augment update.

I have fixed that in patch 3/11 of this series,
which I kept separate for this round of submission
to make it easier for reviewers to see what I
changed there.

However, doing an insert or delete changes the
gap size for the _next_ vma, and potentially a
change in the maximum gap size for the parent
node, so both insert and delete cause two tree
walks :(
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