From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-f72.google.com (mail-ot1-f72.google.com [209.85.210.72]) by kanga.kvack.org (Postfix) with ESMTP id 47BAE6B026D for ; Tue, 9 Oct 2018 09:18:07 -0400 (EDT) Received: by mail-ot1-f72.google.com with SMTP id 36so982807ott.22 for ; Tue, 09 Oct 2018 06:18:07 -0700 (PDT) Received: from foss.arm.com (foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id u107si592747otb.295.2018.10.09.06.18.05 for ; Tue, 09 Oct 2018 06:18:05 -0700 (PDT) Date: Tue, 9 Oct 2018 14:18:04 +0100 From: Will Deacon Subject: Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry Message-ID: <20181009131803.GH6248@arm.com> References: <1539057538-27446-1-git-send-email-anshuman.khandual@arm.com> <20181009130421.bmus632ocurn275u@kshutemo-mobl1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181009130421.bmus632ocurn275u@kshutemo-mobl1> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Anshuman Khandual , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kirill.shutemov@linux.intel.com, akpm@linux-foundation.org, mhocko@suse.com, zi.yan@cs.rutgers.edu On Tue, Oct 09, 2018 at 04:04:21PM +0300, Kirill A. Shutemov wrote: > On Tue, Oct 09, 2018 at 09:28:58AM +0530, Anshuman Khandual wrote: > > A normal mapped THP page at PMD level should be correctly differentiated > > from a PMD migration entry while walking the page table. A mapped THP would > > additionally check positive for pmd_present() along with pmd_trans_huge() > > as compared to a PMD migration entry. This just adds a new conditional test > > differentiating the two while walking the page table. > > > > Fixes: 616b8371539a6 ("mm: thp: enable thp migration in generic path") > > Signed-off-by: Anshuman Khandual > > --- > > On X86, pmd_trans_huge() and is_pmd_migration_entry() are always mutually > > exclusive which makes the current conditional block work for both mapped > > and migration entries. This is not same with arm64 where pmd_trans_huge() > > returns positive for both mapped and migration entries. Could some one > > please explain why pmd_trans_huge() has to return false for migration > > entries which just install swap bits and its still a PMD ? > > I guess it's just a design choice. Any reason why arm64 cannot do the > same? Anshuman, would it work to: #define pmd_trans_huge(pmd) (pmd_present(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT)) ? > > Nonetheless pmd_present() seems to be a better check to distinguish > > between mapped and (non-mapped non-present) migration entries without > > any ambiguity. > > Can we instead reverse order of check: > > if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)) { > pvmw->ptl = pmd_lock(mm, pvmw->pmd); > if (!pmd_present(*pvmw->pmd)) { > ... > } else if (likely(pmd_trans_huge(*pvmw->pmd))) { > ... > } else { > ... > } > ... > > This should cover both imeplementations of pmd_trans_huge(). I'd much rather have portable semantics for pmd_trans_huge(), if we can achieve that efficiently. But that would be fast /and/ correct, so perhaps I'm being too hopeful :) Will