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

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Date:   Tue, 20 Nov 2018 15:12:07 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <OSalvador@...e.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/3] mm, fault_around: do not take a reference to a
 locked page

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@...e.com>
> > 
> > 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?

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