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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS 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 25486C43381 for ; Sun, 17 Mar 2019 13:38:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E277020896 for ; Sun, 17 Mar 2019 13:38:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727136AbfCQNiE (ORCPT ); Sun, 17 Mar 2019 09:38:04 -0400 Received: from szxga07-in.huawei.com ([45.249.212.35]:46430 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726927AbfCQNiE (ORCPT ); Sun, 17 Mar 2019 09:38:04 -0400 Received: from DGGEMS410-HUB.china.huawei.com (unknown [172.30.72.60]) by Forcepoint Email with ESMTP id 33C0F348BBF1B99C3B46; Sun, 17 Mar 2019 21:37:59 +0800 (CST) Received: from [127.0.0.1] (10.184.12.158) by DGGEMS410-HUB.china.huawei.com (10.3.19.210) with Microsoft SMTP Server id 14.3.408.0; Sun, 17 Mar 2019 21:37:50 +0800 Subject: Re: [RFC] Question about TLB flush while set Stage-2 huge pages To: Suzuki K Poulose , CC: , , , , , , , , , , References: <5f712cc6-0874-adbe-add6-46f5de24f36f@huawei.com> <1c0e07b9-73f0-efa4-c1b7-ad81789b42c5@huawei.com> <5188e3b9-5b5a-a6a7-7ef0-09b7b4f06af6@arm.com> <348d0b3b-c74b-7b39-ec30-85905c077c38@huawei.com> <20190314105537.GA15323@en101> <368bd218-ac1d-19b2-6e92-960b91afee8b@huawei.com> <6aea4049-7860-7144-a7be-14f856cdc789@arm.com> From: Zenghui Yu Message-ID: Date: Sun, 17 Mar 2019 21:34:11 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:64.0) Gecko/20100101 Thunderbird/64.0 MIME-Version: 1.0 In-Reply-To: <6aea4049-7860-7144-a7be-14f856cdc789@arm.com> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.184.12.158] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Suzuki, On 2019/3/15 22:56, Suzuki K Poulose wrote: > Hi Zhengui, s/Zhengui/Zheng/ (I think you must wanted to say "Hi" to Zheng :-) ) I have looked into your patch and the kernel log, and I believe that your patch had already addressed this issue. But I think we can do it a little better - two more points need to be handled with caution. Take PMD hugepage (PMD_SIZE == 2M) for example: > > On 15/03/2019 08:21, Zheng Xiang wrote: >> Hi Suzuki, >> >> I have tested this patch, VM doesn't hang and we get expected WARNING >> log: > > Thanks for the quick testing ! > >> However, we also get the following unexpected log: >> >> [  908.329900] BUG: Bad page state in process qemu-kvm  pfn:a2fb41cf >> [  908.339415] page:ffff7e28bed073c0 count:-4 mapcount:0 >> mapping:0000000000000000 index:0x0 >> [  908.339416] flags: 0x4ffffe0000000000() >> [  908.339418] raw: 4ffffe0000000000 dead000000000100 dead000000000200 >> 0000000000000000 >> [  908.339419] raw: 0000000000000000 0000000000000000 fffffffcffffffff >> 0000000000000000 >> [  908.339420] page dumped because: nonzero _refcount >> [  908.339437] CPU: 32 PID: 72599 Comm: qemu-kvm Kdump: loaded >> Tainted: G    B  W        5.0.0+ #1 >> [  908.339438] Call trace: >> [  908.339439]  dump_backtrace+0x0/0x188 >> [  908.339441]  show_stack+0x24/0x30 >> [  908.339442]  dump_stack+0xa8/0xcc >> [  908.339443]  bad_page+0xf0/0x150 >> [  908.339445]  free_pages_check_bad+0x84/0xa0 >> [  908.339446]  free_pcppages_bulk+0x4b8/0x750 >> [  908.339448]  free_unref_page_commit+0x13c/0x198 >> [  908.339449]  free_unref_page+0x84/0xa0 >> [  908.339451]  __free_pages+0x58/0x68 >> [  908.339452]  zap_huge_pmd+0x290/0x2d8 >> [  908.339454]  unmap_page_range+0x2b4/0x470 >> [  908.339455]  unmap_single_vma+0x94/0xe8 >> [  908.339457]  unmap_vmas+0x8c/0x108 >> [  908.339458]  exit_mmap+0xd4/0x178 >> [  908.339459]  mmput+0x74/0x180 >> [  908.339460]  do_exit+0x2b4/0x5b0 >> [  908.339462]  do_group_exit+0x3c/0xe0 >> [  908.339463]  __arm64_sys_exit_group+0x24/0x28 >> [  908.339465]  el0_svc_common+0xa0/0x180 >> [  908.339466]  el0_svc_handler+0x38/0x78 >> [  908.339467]  el0_svc+0x8/0xc > > Thats bad, we seem to be making upto 4 unbalanced put_page(). > >>>> --- >>>>    virt/kvm/arm/mmu.c | 51 >>>> +++++++++++++++++++++++++++++++++++---------------- >>>>    1 file changed, 35 insertions(+), 16 deletions(-) >>>> >>>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c >>>> index 66e0fbb5..04b0f9b 100644 >>>> --- a/virt/kvm/arm/mmu.c >>>> +++ b/virt/kvm/arm/mmu.c >>>> @@ -1076,24 +1076,38 @@ static int stage2_set_pmd_huge(struct kvm >>>> *kvm, struct kvm_mmu_memory_cache >>>>             * Skip updating the page table if the entry is >>>>             * unchanged. >>>>             */ >>>> -        if (pmd_val(old_pmd) == pmd_val(*new_pmd)) >>>> +        if (pmd_val(old_pmd) == pmd_val(*new_pmd)) { >>>>                return 0; >>>> - >>>> +        } else if (WARN_ON_ONCE(!pmd_thp_or_huge(old_pmd))) { >>>>            /* >>>> -         * Mapping in huge pages should only happen through a >>>> -         * fault.  If a page is merged into a transparent huge >>>> -         * page, the individual subpages of that huge page >>>> -         * should be unmapped through MMU notifiers before we >>>> -         * get here. >>>> -         * >>>> -         * Merging of CompoundPages is not supported; they >>>> -         * should become splitting first, unmapped, merged, >>>> -         * and mapped back in on-demand. >>>> +         * If we have PTE level mapping for this block, >>>> +         * we must unmap it to avoid inconsistent TLB >>>> +         * state. We could end up in this situation if >>>> +         * the memory slot was marked for dirty logging >>>> +         * and was reverted, leaving PTE level mappings >>>> +         * for the pages accessed during the period. >>>> +         * Normal THP split/merge follows mmu_notifier >>>> +         * callbacks and do get handled accordingly. >>>>             */ >>>> -        VM_BUG_ON(pmd_pfn(old_pmd) != pmd_pfn(*new_pmd)); >>>> +            unmap_stage2_range(kvm, (addr & S2_PMD_MASK), >>>> S2_PMD_SIZE); First, using unmap_stage2_range() here is not quite appropriate. Suppose we've only accessed one 2M page in HPA [x, x+1]Gib range, with other pages unaccessed. What will happen if unmap_stage2_range(this_2M_page)? We'll unexpectedly reach clear_stage2_pud_entry(), and things are going to get really bad. So we'd better use unmap_stage2_ptes() here since we only want to unmap a 2M range. Second, consider below function stack: unmap_stage2_ptes() clear_stage2_pmd_entry() put_page(virt_to_page(pmd)) It seems that we have one "redundant" put_page() here, (thus comes the bad kernel log ... ,) but actually we do not. By stage2_set_pmd_huge(), the PMD table entry will then point to a 2M block (originally pointed to a PTE table), the _refcount of this PMD-level table page should _not_ change after unmap_stage2_ptes(). So what we really should do is adding a get_page() after unmapping to keep the _refcount a balance! thoughts ? A simple patch below (based on yours) for details. thanks, zenghui >> >> It seems that kvm decreases the _refcount of the page twice in >> transparent_hugepage_adjust() >> and unmap_stage2_range(). > > But I thought we should be doing that on the head_page already, as this > is THP. > I will take a look and get back to you on this. Btw, is it possible for you > to turn on CONFIG_DEBUG_VM and re-run with the above patch ? > > Kind regards > Suzuki > ---8<--- test: kvm: arm: Maybe two more fixes Applied based on Suzuki's patch. Signed-off-by: Zenghui Yu --- virt/kvm/arm/mmu.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c index 05765df..ccd5d5d 100644 --- a/virt/kvm/arm/mmu.c +++ b/virt/kvm/arm/mmu.c @@ -1089,7 +1089,9 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache * Normal THP split/merge follows mmu_notifier * callbacks and do get handled accordingly. */ - unmap_stage2_range(kvm, (addr & S2_PMD_MASK), S2_PMD_SIZE); + addr &= S2_PMD_MASK; + unmap_stage2_ptes(kvm, pmd, addr, addr + S2_PMD_SIZE); + get_page(virt_to_page(pmd)); } else { /* @@ -1138,7 +1140,9 @@ static int stage2_set_pud_huge(struct kvm *kvm, struct kvm_mmu_memory_cache *cac if (stage2_pud_present(kvm, old_pud)) { /* If we have PTE level mapping, unmap the entire range */ if (WARN_ON_ONCE(!stage2_pud_huge(kvm, old_pud))) { - unmap_stage2_range(kvm, addr & S2_PUD_MASK, S2_PUD_SIZE); + addr &= S2_PUD_MASK; + unmap_stage2_pmds(kvm, pudp, addr, addr + S2_PUD_SIZE); + get_page(virt_to_page(pudp)); } else { stage2_pud_clear(kvm, pudp); kvm_tlb_flush_vmid_ipa(kvm, addr); -- 1.8.3.1