Re: [RFC PATCH 3/3] mm, fault_around: do not take a reference to a locked page

[Kirill A. Shutemov]

On Tue, Nov 20, 2018 at 03:12:07PM +0100, Michal Hocko wrote:
> On Tue 20-11-18 17:07:15, Kirill A. Shutemov wrote:
> > On Tue, Nov 20, 2018 at 02:43:23PM +0100, Michal Hocko wrote:
> > > From: Michal Hocko <mhocko@xxxxxxxx>
> > > 
> > > filemap_map_pages takes a speculative reference to each page in the
> > > range before it tries to lock that page. While this is correct it
> > > also can influence page migration which will bail out when seeing
> > > an elevated reference count. The faultaround code would bail on
> > > seeing a locked page so we can pro-actively check the PageLocked
> > > bit before page_cache_get_speculative and prevent from pointless
> > > reference count churn.
> > 
> > Looks fine to me.
> 
> Thanks for the review.
> 
> > But please drop a line of comment in the code. As is it might be confusing
> > for a reader.
> 
> This?

Yep.

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index c76d6a251770..7c4e439a2e85 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2554,6 +2554,10 @@ void filemap_map_pages(struct vm_fault *vmf,
>  
>  		head = compound_head(page);
>  
> +		/*
> +		 * Check the locked pages before taking a reference to not
> +		 * go in the way of migration.
> +		 */
>  		if (PageLocked(head))
>  			goto next;
>  		if (!page_cache_get_speculative(head))
> -- 
> Michal Hocko
> SUSE Labs

-- 
 Kirill A. Shutemov