linux-kernel - Re: [PATCH v2] mm/vmalloc: terminate searching since one node found

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Date:   Mon, 17 Jul 2017 16:45:24 +0800
From:   zijun_hu <zijun_hu@...o.com>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org, zijun_hu@....com,
        zhaoyang.huang@...eadtrum.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Thomas Garnier <thgarnie@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/vmalloc: terminate searching since one node found

On 07/17/2017 04:07 PM, Zhaoyang Huang 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
> 
> 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
>         /   (1)
>       tmp
>        /
>      ...
>       /
>     first(current approach)
> 
> vmap_area_list->...->first->...->tmp->tmp_next

the original code can ensure the following two points :
A, the result vamp_area has the lowest available address in the range [vstart, vend)
B, it can maintain the cached vamp_area node rightly which can speedup relative allocation
i suspect this patch maybe destroy the above two points 
>                             (2)
> 
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@...eadtrum.com>
> ---
>  mm/vmalloc.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 34a1c3e..f833e07 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -459,9 +459,16 @@ 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) {
> +                                       addr = ALIGN(tmp->va_end, align);
> +                                       goto found;
> +                               }
is the aim vamp_area the lowest available one if the goto occurs ?
it will bypass the latter cached vamp_area info  cached_hole_size update possibly if the goto occurs
>                                 if (tmp->va_start <= addr)
>                                         break;
>                                 n = n->rb_left;
> --
> 1.9.1
>