Re: [PATCH -mm 2/7] mm: get unmapped area from VMA tree

[Rik van Riel]

On 06/21/2012 05:01 AM, Johannes Weiner wrote:

> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index bf56d66..8ccb4e1 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -307,6 +307,7 @@ struct mm_struct {
> >   	unsigned long task_size;		/* size of task vm space */
> >   	unsigned long cached_hole_size; 	/* if non-zero, the largest hole below free_area_cache */
> >   	unsigned long free_area_cache;		/* first hole of size cached_hole_size or larger */
> > +	unsigned long highest_vma;		/* highest vma end address */
>
> It's not clear from the name that this is an end address.  Would
> highest_vm_end be better?

Good idea.  Will fix.

> > +	/* Find the left-most free area of sufficient size. */
> > +	for (addr = 0, rb_node = mm->mm_rb.rb_node; rb_node; ) {
> > +		unsigned long vma_start;
> > +		int found_here = 0;
> > +
> > +		vma = rb_to_vma(rb_node);
> > +
> > +		if (vma->vm_start>  len) {
>
> vmas can abut, and vma->vm_end == vma->vm_next->vm_start.  Should this
> be>=?

We do not want to mmap at address 0.

> > +		/* Go left if it looks promising. */
> > +		if (node_free_hole(rb_node->rb_left)>= len&&
> > +					vma->vm_start - len>= lower_limit) {
> > +			rb_node = rb_node->rb_left;
> > +			continue;
>
> If we already are at a vma whose start has a lower address than the
> overall length, does it make sense to check for a left hole?
> I.e. shouldn't this be inside the if (vma->vm_start>  len) block?

I am trying to preserve the same fragmentation
semantics as the current code, so we do not
get any regressions in that area.

> > +	/*
> > +	 * There is not enough space to the left of any VMA.
> > +	 * Check the far right-hand side of the VMA tree.
> > +	 */
> > +	rb_node = mm->mm_rb.rb_node;
> > +	while (rb_node->rb_right)
> > +		rb_node = rb_node->rb_right;
> > +	vma = rb_to_vma(rb_node);
> > +	addr = vma->vm_end;
>
> Unless I missed something, we only reach here when
> continue_next_right(rb_node) above returned NULL.  And if it does, the
> rb_node it was passed was the right-most node in the tree, so we could
> do something like

We break out of the large while() loop once rb_node
is NULL, due to falling off the end of the tree.

> 	} else if (!addr) {
> 		struct rb_node *rb_right = continue_next_right(rb_node);
> 		if (!rb_right)
> 			break;
> 		rb_node = rb_right;
> 		continue;
> 	}
>
> above and then save the lookup after the loop.

That might work, but I expect the situation to be rare
enough that I would rather pick the more readable option.

> Also, dereferencing mm->mm_rb.rb_node unconditionally after the loop
> assumes that the tree always contains at least one vma.  Is this
> guaranteed for all architectures?

When a process is execve'd, a stack VMA is set up.
This means every process has at least one VMA by the
time we can get to this code.

> > -fail:
> > -	/*
> > -	 * if hint left us with no space for the requested
> > -	 * mapping then try again:
> > -	 *
> > -	 * Note: this is different with the case of bottomup
> > -	 * which does the fully line-search, but we use find_vma
> > -	 * here that causes some holes skipped.
> > -	 */
> > -	if (start_addr != mm->mmap_base) {
> > -		mm->free_area_cache = mm->mmap_base;
> > -		mm->cached_hole_size = 0;
> > -		goto try_again;
> > +		if (!found_here&&  node_free_hole(rb_node->rb_left)>= len) {
> > +			/* Last known hole is to the right of this subtree. */
>
> "to the left"

Thanks, will fix.

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