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

--
All rights reversed


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