From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f198.google.com (mail-pg1-f198.google.com [209.85.215.198]) by kanga.kvack.org (Postfix) with ESMTP id B71906B45EB for ; Tue, 28 Aug 2018 11:45:58 -0400 (EDT) Received: by mail-pg1-f198.google.com with SMTP id u6-v6so1370494pgn.10 for ; Tue, 28 Aug 2018 08:45:58 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 14-v6si1338528plb.230.2018.08.28.08.45.57 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 28 Aug 2018 08:45:57 -0700 (PDT) Date: Tue, 28 Aug 2018 17:45:55 +0200 From: Michal Hocko Subject: Re: [PATCH 4/7] mm/hmm: properly handle migration pmd Message-ID: <20180828154555.GS10223@dhcp22.suse.cz> References: <20180824192549.30844-1-jglisse@redhat.com> <20180824192549.30844-5-jglisse@redhat.com> <0560A126-680A-4BAE-8303-F1AB34BE4BA5@cs.rutgers.edu> <20180828152414.GQ10223@dhcp22.suse.cz> <20180828153658.GA4029@redhat.com> <20180828154206.GR10223@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180828154206.GR10223@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Jerome Glisse Cc: Zi Yan , linux-mm@kvack.org, Andrew Morton , linux-kernel@vger.kernel.org, "Aneesh Kumar K . V" , Ralph Campbell , John Hubbard On Tue 28-08-18 17:42:06, Michal Hocko wrote: > On Tue 28-08-18 11:36:59, Jerome Glisse wrote: > > On Tue, Aug 28, 2018 at 05:24:14PM +0200, Michal Hocko wrote: > > > On Fri 24-08-18 20:05:46, Zi Yan wrote: > > > [...] > > > > > + if (!pmd_present(pmd)) { > > > > > + swp_entry_t entry = pmd_to_swp_entry(pmd); > > > > > + > > > > > + if (is_migration_entry(entry)) { > > > > > > > > I think you should check thp_migration_supported() here, since PMD migration is only enabled in x86_64 systems. > > > > Other architectures should treat PMD migration entries as bad. > > > > > > How can we have a migration pmd entry when the migration is not > > > supported? > > > > Not sure i follow here, migration can happen anywhere (assuming > > that something like compaction is active or numa or ...). So this > > code can face pmd migration entry on architecture that support > > it. What is missing here is thp_migration_supported() call to > > protect the is_migration_entry() to avoid false positive on arch > > which do not support thp migration. > > I mean that architectures which do not support THP migration shouldn't > ever see any migration entry. So is_migration_entry should be always > false. Or do I miss something? And just to be clear. thp_migration_supported should be checked only when we actually _do_ the migration or evaluate migratability of the page. We definitely do want to sprinkle this check to all places where is_migration_entry is checked. -- Michal Hocko SUSE Labs