From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752718AbbCGUbG (ORCPT ); Sat, 7 Mar 2015 15:31:06 -0500 Received: from mail-ie0-f181.google.com ([209.85.223.181]:33044 "EHLO mail-ie0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752174AbbCGUbE (ORCPT ); Sat, 7 Mar 2015 15:31:04 -0500 MIME-Version: 1.0 In-Reply-To: <1425741651-29152-2-git-send-email-mgorman@suse.de> References: <1425741651-29152-1-git-send-email-mgorman@suse.de> <1425741651-29152-2-git-send-email-mgorman@suse.de> Date: Sat, 7 Mar 2015 12:31:03 -0800 X-Google-Sender-Auth: Q7D74oXKK6ZrijZItGcz10wrPqk Message-ID: Subject: Re: [PATCH 1/4] mm: thp: Return the correct value for change_huge_pmd From: Linus Torvalds To: Mel Gorman Cc: Dave Chinner , Andrew Morton , Ingo Molnar , Aneesh Kumar , Linux Kernel Mailing List , Linux-MM , xfs@oss.sgi.com, ppc-dev Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Mar 7, 2015 at 7:20 AM, Mel Gorman wrote: > > if (!prot_numa || !pmd_protnone(*pmd)) { > - ret = 1; > entry = pmdp_get_and_clear_notify(mm, addr, pmd); > entry = pmd_modify(entry, newprot); > ret = HPAGE_PMD_NR; Hmm. I know I acked this already, but the return value - which correct - is still potentially something we could improve upon. In particular, we don't need to flush the TLB's if the old entry was not present. Sadly, we don't have a helper function for that. But the code *could* do something like entry = pmdp_get_and_clear_notify(mm, addr, pmd); ret = pmd_tlb_cacheable(entry) ? HPAGE_PMD_NR : 1; entry = pmd_modify(entry, newprot); where pmd_tlb_cacheable() on x86 would test if _PAGE_PRESENT (bit #0) is set. In particular, that would mean that as we change *from* a protnone (whether NUMA or really protnone) we wouldn't need to flush the TLB. In fact, we could make it even more aggressive: it's not just an old non-present TLB entry that doesn't need flushing - we can avoid the flushing whenever we strictly increase the access rigths. So we could have something that takes the old entry _and_ the new protections into account, and avoids the TLB flush if the new entry is strictly more permissive. This doesn't explain the extra TLB flushes Dave sees, though, because the old code didn't make those kinds of optimizations either. But maybe something like this is worth doing. Linus