linux-kernel - Re: [PATCHv5] mm/gup: speed up check_and_migrate_cma

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Date:   Thu, 27 Jun 2019 16:25:11 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     Linux-mm@...ck.org, Ira Weiny <ira.weiny@...el.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Hubbard <jhubbard@...dia.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Christoph Hellwig <hch@....de>,
        Keith Busch <keith.busch@...el.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Linux-kernel@...r.kernel.org
Subject: Re: [PATCHv5] mm/gup: speed up check_and_migrate_cma_pages() on
 huge page

On Thu, 27 Jun 2019 13:15:45 +0800 Pingfan Liu <kernelfans@...il.com> wrote:

> Both hugetlb and thp locate on the same migration type of pageblock, since
> they are allocated from a free_list[]. Based on this fact, it is enough to
> check on a single subpage to decide the migration type of the whole huge
> page. By this way, it saves (2M/4K - 1) times loop for pmd_huge on x86,
> similar on other archs.
> 
> Furthermore, when executing isolate_huge_page(), it avoid taking global
> hugetlb_lock many times, and meanless remove/add to the local link list
> cma_page_list.
>

Thanks, looks good to me.  Have any timing measurements been taken?
 
> ...
>
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1336,25 +1336,30 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
>  					struct vm_area_struct **vmas,
>  					unsigned int gup_flags)
>  {
> -	long i;
> +	long i, step;

I'll make these variables unsigned long - to match nr_pages and because
we have no need for them to be negative.

> ...