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

[David Rientjes <rientjes@google.com>]

On Thu, 26 Apr 2012, Andrew Morton 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?
> 

It happens more often than you may think on page fault tests; how 
representative pft has ever been of actual workloads, especially with thp 
where the benfits of allocating the hugepage usually result in better 
performance in the long-term even for a short-term performance loss, is 
debatable.  However, all other thp code has always dropped 
mm->page_table_lock before calling mem_cgroup_uncharge_page() and this one 
seems to have been missed.  Worth correcting, in my opinion.

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