From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754835AbcFPQsK (ORCPT ); Thu, 16 Jun 2016 12:48:10 -0400 Received: from userp1040.oracle.com ([156.151.31.81]:47689 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753945AbcFPQsI (ORCPT ); Thu, 16 Jun 2016 12:48:08 -0400 Subject: Re: [PATCH] mm: fix account pmd page to the process To: Michal Hocko References: <1466076971-24609-1-git-send-email-zhongjiang@huawei.com> <20160616154214.GA12284@dhcp22.suse.cz> <20160616154324.GN6836@dhcp22.suse.cz> <71df66ac-df29-9542-bfa9-7c94f374df5b@oracle.com> <20160616163119.GP6836@dhcp22.suse.cz> From: Mike Kravetz Cc: zhongjiang , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" Message-ID: Date: Thu, 16 Jun 2016 09:47:46 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.0 MIME-Version: 1.0 In-Reply-To: <20160616163119.GP6836@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Source-IP: userv0022.oracle.com [156.151.31.74] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/16/2016 09:31 AM, Michal Hocko wrote: > On Thu 16-06-16 09:05:23, Mike Kravetz wrote: >> On 06/16/2016 08:43 AM, Michal Hocko wrote: >>> [It seems that this patch has been sent several times and this >>> particular copy didn't add Kirill who has added this code CC him now] >>> >>> On Thu 16-06-16 17:42:14, Michal Hocko wrote: >>>> On Thu 16-06-16 19:36:11, zhongjiang wrote: >>>>> From: zhong jiang >>>>> >>>>> when a process acquire a pmd table shared by other process, we >>>>> increase the account to current process. otherwise, a race result >>>>> in other tasks have set the pud entry. so it no need to increase it. >>>>> >>>>> Signed-off-by: zhong jiang >>>>> --- >>>>> mm/hugetlb.c | 5 ++--- >>>>> 1 file changed, 2 insertions(+), 3 deletions(-) >>>>> >>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>>>> index 19d0d08..3b025c5 100644 >>>>> --- a/mm/hugetlb.c >>>>> +++ b/mm/hugetlb.c >>>>> @@ -4189,10 +4189,9 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud) >>>>> if (pud_none(*pud)) { >>>>> pud_populate(mm, pud, >>>>> (pmd_t *)((unsigned long)spte & PAGE_MASK)); >>>>> - } else { >>>>> + } else >>>>> put_page(virt_to_page(spte)); >>>>> - mm_inc_nr_pmds(mm); >>>>> - } >>>> >>>> The code is quite puzzling but is this correct? Shouldn't we rather do >>>> mm_dec_nr_pmds(mm) in that path to undo the previous inc? >> >> I agree that the code is quite puzzling. :( >> >> However, if this were an issue I would have expected to see some reports. >> Oracle DB makes use of this feature (shared page tables) and if the pmd >> count is wrong we would catch it in check_mm() at exit time. >> >> Upon closer examination, I believe the code in question is never executed. >> Note the callers of huge_pmd_share. The calling code looks like: >> >> if (want_pmd_share() && pud_none(*pud)) >> pte = huge_pmd_share(mm, addr, pud); >> else >> pte = (pte_t *)pmd_alloc(mm, pud, addr); >> >> Therefore, we do not call huge_pmd_share unless pud_none(*pud). The >> code in question is only executed when !pud_none(*pud). > > My understanding is that the check is needed after we retake page lock > because we might have raced with other thread. But it's been quite some > time since I've looked at hugetlb locking and page table sharing code. That is correct, we could have raced. Duh! In the case of a race, the other thread would have incremented the PMD count already. Your suggestion of decrementing pmd count in this case seems to be the correct approach. But, I need to think about this some more. -- Mike Kravetz From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yw0-f199.google.com (mail-yw0-f199.google.com [209.85.161.199]) by kanga.kvack.org (Postfix) with ESMTP id 4239A6B007E for ; Thu, 16 Jun 2016 12:48:03 -0400 (EDT) Received: by mail-yw0-f199.google.com with SMTP id n63so129527992ywf.3 for ; Thu, 16 Jun 2016 09:48:03 -0700 (PDT) Received: from userp1040.oracle.com (userp1040.oracle.com. [156.151.31.81]) by mx.google.com with ESMTPS id n24si27199561qki.131.2016.06.16.09.48.02 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 16 Jun 2016 09:48:02 -0700 (PDT) Subject: Re: [PATCH] mm: fix account pmd page to the process References: <1466076971-24609-1-git-send-email-zhongjiang@huawei.com> <20160616154214.GA12284@dhcp22.suse.cz> <20160616154324.GN6836@dhcp22.suse.cz> <71df66ac-df29-9542-bfa9-7c94f374df5b@oracle.com> <20160616163119.GP6836@dhcp22.suse.cz> From: Mike Kravetz Message-ID: Date: Thu, 16 Jun 2016 09:47:46 -0700 MIME-Version: 1.0 In-Reply-To: <20160616163119.GP6836@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: zhongjiang , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" On 06/16/2016 09:31 AM, Michal Hocko wrote: > On Thu 16-06-16 09:05:23, Mike Kravetz wrote: >> On 06/16/2016 08:43 AM, Michal Hocko wrote: >>> [It seems that this patch has been sent several times and this >>> particular copy didn't add Kirill who has added this code CC him now] >>> >>> On Thu 16-06-16 17:42:14, Michal Hocko wrote: >>>> On Thu 16-06-16 19:36:11, zhongjiang wrote: >>>>> From: zhong jiang >>>>> >>>>> when a process acquire a pmd table shared by other process, we >>>>> increase the account to current process. otherwise, a race result >>>>> in other tasks have set the pud entry. so it no need to increase it. >>>>> >>>>> Signed-off-by: zhong jiang >>>>> --- >>>>> mm/hugetlb.c | 5 ++--- >>>>> 1 file changed, 2 insertions(+), 3 deletions(-) >>>>> >>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>>>> index 19d0d08..3b025c5 100644 >>>>> --- a/mm/hugetlb.c >>>>> +++ b/mm/hugetlb.c >>>>> @@ -4189,10 +4189,9 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud) >>>>> if (pud_none(*pud)) { >>>>> pud_populate(mm, pud, >>>>> (pmd_t *)((unsigned long)spte & PAGE_MASK)); >>>>> - } else { >>>>> + } else >>>>> put_page(virt_to_page(spte)); >>>>> - mm_inc_nr_pmds(mm); >>>>> - } >>>> >>>> The code is quite puzzling but is this correct? Shouldn't we rather do >>>> mm_dec_nr_pmds(mm) in that path to undo the previous inc? >> >> I agree that the code is quite puzzling. :( >> >> However, if this were an issue I would have expected to see some reports. >> Oracle DB makes use of this feature (shared page tables) and if the pmd >> count is wrong we would catch it in check_mm() at exit time. >> >> Upon closer examination, I believe the code in question is never executed. >> Note the callers of huge_pmd_share. The calling code looks like: >> >> if (want_pmd_share() && pud_none(*pud)) >> pte = huge_pmd_share(mm, addr, pud); >> else >> pte = (pte_t *)pmd_alloc(mm, pud, addr); >> >> Therefore, we do not call huge_pmd_share unless pud_none(*pud). The >> code in question is only executed when !pud_none(*pud). > > My understanding is that the check is needed after we retake page lock > because we might have raced with other thread. But it's been quite some > time since I've looked at hugetlb locking and page table sharing code. That is correct, we could have raced. Duh! In the case of a race, the other thread would have incremented the PMD count already. Your suggestion of decrementing pmd count in this case seems to be the correct approach. But, I need to think about this some more. -- Mike Kravetz -- 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