On 07/18/2017 04:31 PM, Zhaoyang Huang (黄朝阳) wrote:
>
> It is no need to find the very beginning of the area within
> alloc_vmap_area, which can be done by judging each node during the process
>
it seems the original code is wrote to achieve the following two purposes :
A, the result vamp_area has the lowest available address in the required range [vstart, vend)
B, it maybe update the cached vamp_area node info which can speedup other relative allocations
it look redundant but conventional and necessary
this approach maybe destroy the original purposes
> For current approach, the worst case is that the starting node which be found
> for searching the 'vmap_area_list' is close to the 'vstart', while the final
> available one is round to the tail(especially for the left branch).
> This commit have the list searching start at the first available node, which
> will save the time of walking the rb tree'(1)' and walking the list'(2)'.
>
> vmap_area_root
> / \
> tmp_next U
> /
> tmp
> /
> ... (1)
> /
> first(current approach)
>
@tmp_next is the next node of @tmp in the ordered list_head, not in the rbtree
> vmap_area_list->...->first->...->tmp->tmp_next
> (2)
>
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxxxxxx>
> ---
> mm/vmalloc.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 34a1c3e..9a5c177 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -459,9 +459,18 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>
> while (n) {
> struct vmap_area *tmp;
> + struct vmap_area *tmp_next;
> tmp = rb_entry(n, struct vmap_area, rb_node);
> + tmp_next = list_next_entry(tmp, list);
> if (tmp->va_end >= addr) {
> first = tmp;
> + if (ALIGN(tmp->va_end, align) + size
> + < tmp_next->va_start) {
if @tmp node don't locate in the required rang [vstart, vend), but the right of the range it maybe
satisfy this condition, even if it locate it locate within the range, it maybe don't have the lowest free address.
if @tmp don't have the next node, tmp_next->va_start will cause NULL dereference
> + addr = ALIGN(tmp->va_end, align);
> + if (cached_hole_size >= size)
> + cached_hole_size = 0;
it seems a little rough to reset the @cached_hole_size by this way, it will caused the cached info is updated in the next
allocation regardless the allocation arguments.
> + goto found;
> + }
> if (tmp->va_start <= addr)
> break;
> n = n->rb_left;
> --
> 1.9.1
>
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