From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3228FC43334 for ; Thu, 6 Sep 2018 14:17:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D7E6120844 for ; Thu, 6 Sep 2018 14:17:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D7E6120844 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729977AbeIFSxB (ORCPT ); Thu, 6 Sep 2018 14:53:01 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:46168 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728905AbeIFSxA (ORCPT ); Thu, 6 Sep 2018 14:53:00 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 57D2440241DC; Thu, 6 Sep 2018 14:17:14 +0000 (UTC) Received: from redhat.com (ovpn-123-203.rdu2.redhat.com [10.10.123.203]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 80AF12027EA0; Thu, 6 Sep 2018 14:17:10 +0000 (UTC) Date: Thu, 6 Sep 2018 10:17:09 -0400 From: Jerome Glisse To: Peter Xu Cc: "Kirill A. Shutemov" , Zi Yan , linux-kernel@vger.kernel.org, Andrea Arcangeli , Andrew Morton , Michal Hocko , Huang Ying , Dan Williams , Naoya Horiguchi , "Aneesh Kumar K.V" , Konstantin Khlebnikov , Souptick Joarder , linux-mm@kvack.org Subject: Re: [PATCH] mm: hugepage: mark splitted page dirty when needed Message-ID: <20180906141708.GB3830@redhat.com> References: <20180904075510.22338-1-peterx@redhat.com> <20180904080115.o2zj4mlo7yzjdqfl@kshutemo-mobl1> <20180905073037.GA23021@xz-x1> <20180905125522.x2puwfn5sr2zo3go@kshutemo-mobl1> <20180906113933.GG16937@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180906113933.GG16937@xz-x1> User-Agent: Mutt/1.10.0 (2018-05-17) X-Scanned-By: MIMEDefang 2.78 on 10.11.54.4 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.7]); Thu, 06 Sep 2018 14:17:14 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.7]); Thu, 06 Sep 2018 14:17:14 +0000 (UTC) for IP:'10.11.54.4' DOMAIN:'int-mx04.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'jglisse@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 06, 2018 at 07:39:33PM +0800, Peter Xu wrote: > On Wed, Sep 05, 2018 at 03:55:22PM +0300, Kirill A. Shutemov wrote: > > On Wed, Sep 05, 2018 at 03:30:37PM +0800, Peter Xu wrote: > > > On Tue, Sep 04, 2018 at 10:00:28AM -0400, Zi Yan wrote: > > > > On 4 Sep 2018, at 4:01, Kirill A. Shutemov wrote: > > > > > > > > > On Tue, Sep 04, 2018 at 03:55:10PM +0800, Peter Xu wrote: > > > > >> When splitting a huge page, we should set all small pages as dirty if > > > > >> the original huge page has the dirty bit set before. Otherwise we'll > > > > >> lose the original dirty bit. > > > > > > > > > > We don't lose it. It got transfered to struct page flag: > > > > > > > > > > if (pmd_dirty(old_pmd)) > > > > > SetPageDirty(page); > > > > > > > > > > > > > Plus, when split_huge_page_to_list() splits a THP, its subroutine __split_huge_page() > > > > propagates the dirty bit in the head page flag to all subpages in __split_huge_page_tail(). > > > > > > Hi, Kirill, Zi, > > > > > > Thanks for your responses! > > > > > > Though in my test the huge page seems to be splitted not by > > > split_huge_page_to_list() but by explicit calls to > > > change_protection(). The stack looks like this (again, this is a > > > customized kernel, and I added an explicit dump_stack() there): > > > > > > kernel: dump_stack+0x5c/0x7b > > > kernel: __split_huge_pmd+0x192/0xdc0 > > > kernel: ? update_load_avg+0x8b/0x550 > > > kernel: ? update_load_avg+0x8b/0x550 > > > kernel: ? account_entity_enqueue+0xc5/0xf0 > > > kernel: ? enqueue_entity+0x112/0x650 > > > kernel: change_protection+0x3a2/0xab0 > > > kernel: mwriteprotect_range+0xdd/0x110 > > > kernel: userfaultfd_ioctl+0x50b/0x1210 > > > kernel: ? do_futex+0x2cf/0xb20 > > > kernel: ? tty_write+0x1d2/0x2f0 > > > kernel: ? do_vfs_ioctl+0x9f/0x610 > > > kernel: do_vfs_ioctl+0x9f/0x610 > > > kernel: ? __x64_sys_futex+0x88/0x180 > > > kernel: ksys_ioctl+0x70/0x80 > > > kernel: __x64_sys_ioctl+0x16/0x20 > > > kernel: do_syscall_64+0x55/0x150 > > > kernel: entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > > > At the very time the userspace is sending an UFFDIO_WRITEPROTECT ioctl > > > to kernel space, which is handled by mwriteprotect_range(). In case > > > you'd like to refer to the kernel, it's basically this one from > > > Andrea's (with very trivial changes): > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git userfault > > > > > > So... do we have two paths to split the huge pages separately? > > > > We have two entiries that can be split: page table enties and underlying > > compound page. > > > > split_huge_pmd() (and variants of it) split the PMD entry into a PTE page > > table. It doens't touch underlying compound page. The page still can be > > mapped in other place as huge. > > > > split_huge_page() (and ivariants of it) split compound page into a number > > of 4k (or whatever PAGE_SIZE is). The operation requires splitting all > > PMD, but not other way around. > > > > > > > > Another (possibly very naive) question is: could any of you hint me > > > how the page dirty bit is finally applied to the PTEs? These two > > > dirty flags confused me for a few days already (the SetPageDirty() one > > > which sets the page dirty flag, and the pte_mkdirty() which sets that > > > onto the real PTEs). > > > > Dirty bit from page table entries transferes to sturct page flug and used > > for decision making in reclaim path. > > Thanks for explaining. It's much clearer for me. > > Though for the issue I have encountered, I am still confused on why > that dirty bit can be ignored for the splitted PTEs. Indeed we have: > > if (pmd_dirty(old_pmd)) > SetPageDirty(page); > > However to me this only transfers (as you explained above) the dirty > bit (AFAIU it's possibly set by the hardware when the page is written) > to the page struct of the compound page. It did not really apply to > every small page of the splitted huge page. As you also explained, > this __split_huge_pmd() only splits the PMD entry but it keeps the > compound huge page there, then IMHO it should also apply the dirty > bits from the huge page to all the small page entries, no? > > These dirty bits are really important to my scenario since AFAIU the > change_protection() call is using these dirty bits to decide whether > it should append the WRITE bit - it finally corresponds to the lines > in change_pte_range(): > > /* Avoid taking write faults for known dirty pages */ > if (dirty_accountable && pte_dirty(ptent) && > (pte_soft_dirty(ptent) || > !(vma->vm_flags & VM_SOFTDIRTY))) { > ptent = pte_mkwrite(ptent); > } > > So when mprotect() with that range (my case is UFFDIO_WRITEPROTECT, > which is similar) although we pass in the new protocol with VM_WRITE > here it'll still mask it since the dirty bit is not set, then the > userspace program (in my case, the QEMU thread that handles write > protect failures) can never fixup the write-protected page fault. > > Am I missing anything important here? > For reference mwriteprotect_range code: https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?id=b16cb9fcb76bec59cbe1427e73246dc81a4942e2 mwriteprotect_range usage: https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?id=aa97daa6e54f2cfed1a6f1f38f9629608b8aadcc Maybe you can describe the issues you are having because i admit not seing what is wrong here. When mwriteprotect_range is call with UFFDIO_WRITEPROTECT_MODE_WP then dirty_accountable is false and thus above if is not taken and pte is properly write protected and thus UFFDIO_WRITEPROTECT_MODE_WP do what its name suggest no matter what is the pte dirty state. I am not sure what UFFDIO_WRITEPROTECT_MODE_DONTWAKE means as this is the one that might depends on the pte dirty state. So without knowing what UFFDIO_WRITEPROTECT_MODE_DONTWAKE do, i am not sure i see any bug here. Cheers, Jérôme From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f200.google.com (mail-qk1-f200.google.com [209.85.222.200]) by kanga.kvack.org (Postfix) with ESMTP id BDDC46B78F9 for ; Thu, 6 Sep 2018 10:17:17 -0400 (EDT) Received: by mail-qk1-f200.google.com with SMTP id w126-v6so7838997qka.11 for ; Thu, 06 Sep 2018 07:17:17 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id y27-v6si427069qtc.394.2018.09.06.07.17.14 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 06 Sep 2018 07:17:14 -0700 (PDT) Date: Thu, 6 Sep 2018 10:17:09 -0400 From: Jerome Glisse Subject: Re: [PATCH] mm: hugepage: mark splitted page dirty when needed Message-ID: <20180906141708.GB3830@redhat.com> References: <20180904075510.22338-1-peterx@redhat.com> <20180904080115.o2zj4mlo7yzjdqfl@kshutemo-mobl1> <20180905073037.GA23021@xz-x1> <20180905125522.x2puwfn5sr2zo3go@kshutemo-mobl1> <20180906113933.GG16937@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180906113933.GG16937@xz-x1> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Xu Cc: "Kirill A. Shutemov" , Zi Yan , linux-kernel@vger.kernel.org, Andrea Arcangeli , Andrew Morton , Michal Hocko , Huang Ying , Dan Williams , Naoya Horiguchi , "Aneesh Kumar K.V" , Konstantin Khlebnikov , Souptick Joarder , linux-mm@kvack.org On Thu, Sep 06, 2018 at 07:39:33PM +0800, Peter Xu wrote: > On Wed, Sep 05, 2018 at 03:55:22PM +0300, Kirill A. Shutemov wrote: > > On Wed, Sep 05, 2018 at 03:30:37PM +0800, Peter Xu wrote: > > > On Tue, Sep 04, 2018 at 10:00:28AM -0400, Zi Yan wrote: > > > > On 4 Sep 2018, at 4:01, Kirill A. Shutemov wrote: > > > > > > > > > On Tue, Sep 04, 2018 at 03:55:10PM +0800, Peter Xu wrote: > > > > >> When splitting a huge page, we should set all small pages as dirty if > > > > >> the original huge page has the dirty bit set before. Otherwise we'll > > > > >> lose the original dirty bit. > > > > > > > > > > We don't lose it. It got transfered to struct page flag: > > > > > > > > > > if (pmd_dirty(old_pmd)) > > > > > SetPageDirty(page); > > > > > > > > > > > > > Plus, when split_huge_page_to_list() splits a THP, its subroutine __split_huge_page() > > > > propagates the dirty bit in the head page flag to all subpages in __split_huge_page_tail(). > > > > > > Hi, Kirill, Zi, > > > > > > Thanks for your responses! > > > > > > Though in my test the huge page seems to be splitted not by > > > split_huge_page_to_list() but by explicit calls to > > > change_protection(). The stack looks like this (again, this is a > > > customized kernel, and I added an explicit dump_stack() there): > > > > > > kernel: dump_stack+0x5c/0x7b > > > kernel: __split_huge_pmd+0x192/0xdc0 > > > kernel: ? update_load_avg+0x8b/0x550 > > > kernel: ? update_load_avg+0x8b/0x550 > > > kernel: ? account_entity_enqueue+0xc5/0xf0 > > > kernel: ? enqueue_entity+0x112/0x650 > > > kernel: change_protection+0x3a2/0xab0 > > > kernel: mwriteprotect_range+0xdd/0x110 > > > kernel: userfaultfd_ioctl+0x50b/0x1210 > > > kernel: ? do_futex+0x2cf/0xb20 > > > kernel: ? tty_write+0x1d2/0x2f0 > > > kernel: ? do_vfs_ioctl+0x9f/0x610 > > > kernel: do_vfs_ioctl+0x9f/0x610 > > > kernel: ? __x64_sys_futex+0x88/0x180 > > > kernel: ksys_ioctl+0x70/0x80 > > > kernel: __x64_sys_ioctl+0x16/0x20 > > > kernel: do_syscall_64+0x55/0x150 > > > kernel: entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > > > At the very time the userspace is sending an UFFDIO_WRITEPROTECT ioctl > > > to kernel space, which is handled by mwriteprotect_range(). In case > > > you'd like to refer to the kernel, it's basically this one from > > > Andrea's (with very trivial changes): > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git userfault > > > > > > So... do we have two paths to split the huge pages separately? > > > > We have two entiries that can be split: page table enties and underlying > > compound page. > > > > split_huge_pmd() (and variants of it) split the PMD entry into a PTE page > > table. It doens't touch underlying compound page. The page still can be > > mapped in other place as huge. > > > > split_huge_page() (and ivariants of it) split compound page into a number > > of 4k (or whatever PAGE_SIZE is). The operation requires splitting all > > PMD, but not other way around. > > > > > > > > Another (possibly very naive) question is: could any of you hint me > > > how the page dirty bit is finally applied to the PTEs? These two > > > dirty flags confused me for a few days already (the SetPageDirty() one > > > which sets the page dirty flag, and the pte_mkdirty() which sets that > > > onto the real PTEs). > > > > Dirty bit from page table entries transferes to sturct page flug and used > > for decision making in reclaim path. > > Thanks for explaining. It's much clearer for me. > > Though for the issue I have encountered, I am still confused on why > that dirty bit can be ignored for the splitted PTEs. Indeed we have: > > if (pmd_dirty(old_pmd)) > SetPageDirty(page); > > However to me this only transfers (as you explained above) the dirty > bit (AFAIU it's possibly set by the hardware when the page is written) > to the page struct of the compound page. It did not really apply to > every small page of the splitted huge page. As you also explained, > this __split_huge_pmd() only splits the PMD entry but it keeps the > compound huge page there, then IMHO it should also apply the dirty > bits from the huge page to all the small page entries, no? > > These dirty bits are really important to my scenario since AFAIU the > change_protection() call is using these dirty bits to decide whether > it should append the WRITE bit - it finally corresponds to the lines > in change_pte_range(): > > /* Avoid taking write faults for known dirty pages */ > if (dirty_accountable && pte_dirty(ptent) && > (pte_soft_dirty(ptent) || > !(vma->vm_flags & VM_SOFTDIRTY))) { > ptent = pte_mkwrite(ptent); > } > > So when mprotect() with that range (my case is UFFDIO_WRITEPROTECT, > which is similar) although we pass in the new protocol with VM_WRITE > here it'll still mask it since the dirty bit is not set, then the > userspace program (in my case, the QEMU thread that handles write > protect failures) can never fixup the write-protected page fault. > > Am I missing anything important here? > For reference mwriteprotect_range code: https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?id=b16cb9fcb76bec59cbe1427e73246dc81a4942e2 mwriteprotect_range usage: https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?id=aa97daa6e54f2cfed1a6f1f38f9629608b8aadcc Maybe you can describe the issues you are having because i admit not seing what is wrong here. When mwriteprotect_range is call with UFFDIO_WRITEPROTECT_MODE_WP then dirty_accountable is false and thus above if is not taken and pte is properly write protected and thus UFFDIO_WRITEPROTECT_MODE_WP do what its name suggest no matter what is the pte dirty state. I am not sure what UFFDIO_WRITEPROTECT_MODE_DONTWAKE means as this is the one that might depends on the pte dirty state. So without knowing what UFFDIO_WRITEPROTECT_MODE_DONTWAKE do, i am not sure i see any bug here. Cheers, Jerome