Re: [PATCH resend] mm: hugetlb_vmemmap: use bulk allocator in alloc_vmemmap_page_list()

[Muchun Song <muchun.song@linux.dev>]

> On Sep 6, 2023, at 17:33, Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
> 
> 
> 
> On 2023/9/6 11:25, Muchun Song wrote:
>>> On Sep 6, 2023, at 11:13, Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>>> 
>>> 
>>> 
>>> On 2023/9/6 10:47, Matthew Wilcox wrote:
>>>> On Tue, Sep 05, 2023 at 06:35:08PM +0800, Kefeng Wang wrote:
>>>>> It is needed 4095 pages(1G) or 7 pages(2M) to be allocated once in
>>>>> alloc_vmemmap_page_list(), so let's add a bulk allocator varietas
>>>>> alloc_pages_bulk_list_node() and switch alloc_vmemmap_page_list()
>>>>> to use it to accelerate page allocation.
>>>> Argh, no, please don't do this.
>>>> Iterating a linked list is _expensive_.  It is about 10x quicker to
>>>> iterate an array than a linked list.  Adding the list_head option
>>>> to __alloc_pages_bulk() was a colossal mistake.  Don't perpetuate it.
>>>> These pages are going into an array anyway.  Don't put them on a list
>>>> first.
>>> 
>>> struct vmemmap_remap_walk - walk vmemmap page table
>>> 
>>> * @vmemmap_pages:  the list head of the vmemmap pages that can be freed
>>> *                  or is mapped from.
>>> 
>>> At present, the struct vmemmap_remap_walk use a list for vmemmap page table walk, so do you mean we need change vmemmap_pages from a list to a array firstly and then use array bulk api, even kill list bulk api ?
>> It'll be a little complex for hugetlb_vmemmap. Should it be reasonable to
>> directly use __alloc_pages_bulk in hugetlb_vmemmap itself?
> 
> 
> We could use alloc_pages_bulk_array_node() here without introduce a new
> alloc_pages_bulk_list_node(), only focus on accelerate page allocation
> for now.
> 

No. Using alloc_pages_bulk_array_node() will add more complexity (you need to allocate
an array fist) for hugetlb_vmemap and this path that you optimized is only a control
path and this optimization is at the millisecond level. So I don't think it is a great
value to do this.

Thanks.