From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qa0-f41.google.com (mail-qa0-f41.google.com [209.85.216.41]) by kanga.kvack.org (Postfix) with ESMTP id CA1566B0035 for ; Thu, 6 Mar 2014 17:57:36 -0500 (EST) Received: by mail-qa0-f41.google.com with SMTP id j5so3328865qaq.14 for ; Thu, 06 Mar 2014 14:57:36 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTP id v2si3984615qaf.6.2014.03.06.14.57.35 for ; Thu, 06 Mar 2014 14:57:36 -0800 (PST) Message-ID: <5318FC3F.4080204@redhat.com> Date: Thu, 06 Mar 2014 17:52:47 -0500 From: Rik van Riel MIME-Version: 1.0 Subject: Re: [PATCH -mm] mm,numa,mprotect: always continue after finding a stable thp page References: <5318E4BC.50301@oracle.com> <20140306173137.6a23a0b2@cuia.bos.redhat.com> In-Reply-To: <20140306173137.6a23a0b2@cuia.bos.redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Sasha Levin Cc: David Rientjes , Andrew Morton , "linux-mm@kvack.org" , LKML , mgorman@suse.de, hhuang@redhat.com, knoel@redhat.com, aarcange@redhat.com On 03/06/2014 05:31 PM, Rik van Riel wrote: > On Thu, 06 Mar 2014 16:12:28 -0500 > Sasha Levin wrote: > >> While fuzzing with trinity inside a KVM tools guest running latest -next kernel I've hit the >> following spew. This seems to be introduced by your patch "mm,numa: reorganize change_pmd_range()". > > That patch should not introduce any functional changes, except for > the VM_BUG_ON that catches the fact that we fell through to the 4kB > pte handling code, despite having just handled a THP pmd... > > Does this patch fix the issue? > > Mel, am I overlooking anything obvious? :) > > ---8<--- > > Subject: mm,numa,mprotect: always continue after finding a stable thp page > > When turning a thp pmds into a NUMA one, change_huge_pmd will > return 0 when the pmd already is a NUMA pmd. I did miss something obvious. In this case, the code returns 1. > However, change_pmd_range would fall through to the code that > handles 4kB pages, instead of continuing on to the next pmd. Maybe the case that I missed is when khugepaged is in the process of collapsing pages into a transparent huge page? If the virtual CPU gets de-scheduled by the host for long enough, it would be possible for khugepaged to run on another virtual CPU, and turn the pmd into a THP pmd, before that VM_BUG_ON test. I see that khugepaged takes the mmap_sem for writing in the collapse code, and it looks like task_numa_work takes the mmap_sem for reading, so I guess that may not be possible? Andrea, would you happen to know what case am I missing? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org