Re: [patch] mm, thp: drop page_table_lock to uncharge memcg pages

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 26 Apr 2012 15:57:30 -0700 (PDT)
David Rientjes <rientjes@google.com> wrote:

> mm->page_table_lock is hotly contested for page fault tests and isn't
> necessary to do mem_cgroup_uncharge_page() in do_huge_pmd_wp_page().
> 
> ...
>
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -968,8 +968,10 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  	spin_lock(&mm->page_table_lock);
>  	put_page(page);
>  	if (unlikely(!pmd_same(*pmd, orig_pmd))) {
> +		spin_unlock(&mm->page_table_lock);
>  		mem_cgroup_uncharge_page(new_page);
>  		put_page(new_page);
> +		goto out;
>  	} else {
>  		pmd_t entry;
>  		VM_BUG_ON(!PageHead(page));

But this is on the basically-never-happens race path and will surely have no
measurable benefit?

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