linux-kernel - Re: [PATCH -mm 1/7] mm: track free size between VMAs in VMA rbtree

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Date:	Tue, 19 Jun 2012 16:25:47 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-mm@...ck.org, aarcange@...hat.com, peterz@...radead.org,
	minchan@...il.com, kosaki.motohiro@...il.com, andi@...stfloor.org,
	hannes@...xchg.org, mel@....ul.ie, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH -mm 1/7] mm: track free size between VMAs in VMA rbtree

On Mon, 18 Jun 2012 18:05:20 -0400
Rik van Riel <riel@...hat.com> wrote:

> From: Rik van Riel <riel@...riel.com>
> 
> Track the size of free areas between VMAs in the VMA rbtree.
> 
> This will allow get_unmapped_area_* to find a free area of the
> right size in O(log(N)) time, instead of potentially having to
> do a linear walk across all the VMAs.
> 
> ...
>
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -213,6 +213,13 @@ struct vm_area_struct {
>  	struct rb_node vm_rb;
>  
>  	/*
> +	 * Largest free memory gap "behind" this VMA (in the direction mmap
> +	 * grows from), or of VMAs down the rb tree below us. This helps
> +	 * get_unmapped_area find a free area of the right size.
> +	 */
> +	unsigned long free_gap;

Please mention the units?  Seems to be "bytes", not "pages".

> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -205,6 +205,51 @@ static void __remove_shared_vm_struct(struct vm_area_struct *vma,
>  	flush_dcache_mmap_unlock(mapping);
>  }
>  
> +static unsigned long max_free_space(struct rb_node *node)
> +{
> +	struct vm_area_struct *vma, *prev, *left = NULL, *right = NULL;
> +	unsigned long largest = 0;
> +
> +	if (node->rb_left)
> +		left = rb_to_vma(node->rb_left);
> +	if (node->rb_right)
> +		right = rb_to_vma(node->rb_right);
> +
> +	/*
> +	 * Calculate the free gap size between us and the
> +	 * VMA to our left.
> +	 */

Comment will fit in a single line.

> +	vma = rb_to_vma(node);
> +	prev = vma->vm_prev;
> +
> +	if (prev)
> +		largest = vma->vm_start - prev->vm_end;
> +	else
> +		largest = vma->vm_start;
> +
> +	/* We propagate the largest of our own, or our children's free gaps. */
> +	if (left)
> +		largest = max(largest, left->free_gap);
> +	if (right)
> +		largest = max(largest, right->free_gap);
> +
> +	return largest;
> +}

This would be easier to review if it had a nice comment explaining its
role ;)

> +static void vma_rb_augment_cb(struct rb_node *node, void *__unused)
> +{
> +	struct vm_area_struct *vma;
> +
> +	vma = rb_to_vma(node);
> +
> +	vma->free_gap = max_free_space(node);
> +}

Save some trees!

	struct vm_area_struct *vma = rb_to_vma(node);
	vma->free_gap = max_free_space(node);

or even

	rb_to_vma(node)->free_gap = max_free_space(node);


Major stuff, huh?
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