From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id 98CB16B2064 for ; Tue, 20 Nov 2018 09:12:10 -0500 (EST) Received: by mail-ed1-f72.google.com with SMTP id c3so1369489eda.3 for ; Tue, 20 Nov 2018 06:12:10 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id p10si83007edh.181.2018.11.20.06.12.09 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 20 Nov 2018 06:12:09 -0800 (PST) Date: Tue, 20 Nov 2018 15:12:07 +0100 From: Michal Hocko Subject: Re: [RFC PATCH 3/3] mm, fault_around: do not take a reference to a locked page Message-ID: <20181120141207.GK22247@dhcp22.suse.cz> References: <20181120134323.13007-1-mhocko@kernel.org> <20181120134323.13007-4-mhocko@kernel.org> <20181120140715.mouc7okin3ht5krr@kshutemo-mobl1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181120140715.mouc7okin3ht5krr@kshutemo-mobl1> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: linux-mm@kvack.org, Andrew Morton , Oscar Salvador , Pavel Tatashin , David Hildenbrand , LKML 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 > > > > 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