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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3A169C43334 for ; Mon, 13 Jun 2022 20:10:24 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 8F7956B0301; Mon, 13 Jun 2022 16:10:23 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 8A7C88D01C0; Mon, 13 Jun 2022 16:10:23 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 7215E8D01BF; Mon, 13 Jun 2022 16:10:23 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id 5E5206B0301 for ; Mon, 13 Jun 2022 16:10:23 -0400 (EDT) Received: from smtpin08.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 31A8120DEC for ; Mon, 13 Jun 2022 20:10:23 +0000 (UTC) X-FDA: 79574304726.08.67C0D7D Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by imf16.hostedemail.com (Postfix) with ESMTP id 8A1C2180083 for ; Mon, 13 Jun 2022 20:10:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1655151022; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=rR7JaD3q8qTUeYECk93qYjKc7dif3t1p4wuEQFyTh6k=; b=GkHJIfSfRIWTUiqPnHFGyBMTvkXxQWIqFfJk+k8TTwedUcdaxttYDTOLAcv4YT5uO2Jikk S/46GrupC0vuoNd9fIgKHF1VzUHjOUFiY6Z3PGtN+F4qjTwuK3HPXDXJCyWdV4+y0FCdAU nRj3wbE7h+oS0denIUOeZP0BQomQqno= Received: from mail-wm1-f69.google.com (mail-wm1-f69.google.com [209.85.128.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-208-uLkrX7XOO72-A4Lfya8Caw-1; Mon, 13 Jun 2022 16:10:19 -0400 X-MC-Unique: uLkrX7XOO72-A4Lfya8Caw-1 Received: by mail-wm1-f69.google.com with SMTP id k34-20020a05600c1ca200b0039c7db490c8so3726930wms.1 for ; Mon, 13 Jun 2022 13:10:19 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent :content-language:to:cc:references:from:organization:subject :in-reply-to:content-transfer-encoding; bh=rR7JaD3q8qTUeYECk93qYjKc7dif3t1p4wuEQFyTh6k=; b=rpqx62bvpNO00E8pJM2eV0XqZvNixCPfw4l+B+phEBRugRLx5Efaqbo12rDQxJ0gkN rU0/jhHIlIsBW/5TLNIiKizHHQTJ9roNYouy8jMfe62x6OMnPRNlov7442F+xIike044 skrxvVHcAx+8h95D4kg4n1OdQDZjErnb//3PAYhC64mOJRRErdVyyaUJ8nKHnehqSC7b ID2IFjXpXJrPg4bS6amDA9uMsov0mvvQ5DWm3yl2SNSsjGtSXMqXJhr6cAupVsjIEJaL lxxJm3QcvVUIk0H5teLvKuLLmthqaX7xUz1doN9wnarY5Ou5yGkeCgkLNCz3TWyzwcx3 s5hQ== X-Gm-Message-State: AJIora8Fwv17zu2dlHR9vLm885RstJrZ8GQR59vPQn8EgNLnK/QQ1osI QVLx3iCOM8NGQAVSvqn+8/asrEdnMP+Csju5C6pOC1EvBx/dqmIP30cfuHTGofkvxgql4J+G/Pn 0+4JHfRbbp38= X-Received: by 2002:adf:ae09:0:b0:20e:e4f0:2133 with SMTP id x9-20020adfae09000000b0020ee4f02133mr1429652wrc.104.1655151018013; Mon, 13 Jun 2022 13:10:18 -0700 (PDT) X-Google-Smtp-Source: AGRyM1tEBfGYzbRoGtpWCbNt96IUnAi4iF5SlIbS1OLPQdVbI1Q9kTbvPd7uMfJeVRnhpsdg5LvrYQ== X-Received: by 2002:adf:ae09:0:b0:20e:e4f0:2133 with SMTP id x9-20020adfae09000000b0020ee4f02133mr1429633wrc.104.1655151017734; Mon, 13 Jun 2022 13:10:17 -0700 (PDT) Received: from ?IPV6:2003:cb:c706:bd00:963c:5455:c10e:fa6f? (p200300cbc706bd00963c5455c10efa6f.dip0.t-ipconnect.de. [2003:cb:c706:bd00:963c:5455:c10e:fa6f]) by smtp.gmail.com with ESMTPSA id x20-20020a1c7c14000000b0039c506de087sm10670867wmc.6.2022.06.13.13.10.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 13 Jun 2022 13:10:17 -0700 (PDT) Message-ID: <9b38302c-ed93-8825-f543-6ce8878748f9@redhat.com> Date: Mon, 13 Jun 2022 22:10:16 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 To: Peter Xu , Nadav Amit Cc: LKML , linux-mm , Linus Torvalds , Andrew Morton , Dave Hansen , Andrea Arcangeli , Yang Shi , Hugh Dickins , Mel Gorman , Peter Collingbourne References: <20220610181436.84713-1-david@redhat.com> <5DFB7262-6E32-4984-A346-B7DE5040B12F@gmail.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v2] mm/mprotect: try avoiding write faults for exclusive anonymous pages when changing protection In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1655151022; a=rsa-sha256; cv=none; b=KnDQsRGlbaug/xIZhMVAVNNmmMgy/+jbZE9U089W4JznQoXS7kxNnGwDBnsZkjBeaZFwai +tmiIMRdMX+tm2myVHDLb/+qVnK0/J7eDgokx7fzmRUiOpRhH1cHVecV7oYjBQfuZM7Myg mEnNZ7x41tsYyQq31vdY3awul+hc6bw= ARC-Authentication-Results: i=1; imf16.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=GkHJIfSf; spf=none (imf16.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1655151022; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=rR7JaD3q8qTUeYECk93qYjKc7dif3t1p4wuEQFyTh6k=; b=VLXNZJifsDbv2EFn33t9YaAmctDjwg3yrjrU+pKnv8NoEu7/c+Wu/qBg/BDQYyvVRwJYax Sb3JZqY/lrxCtMRUsJm1+mQXoJ4pPkEt+NfkffipCpSyV/FXqobVVqXnwPZn/f4baq6U7M vgTJLWi+Tfna9ie5gTqDO/GotZphqfM= X-Stat-Signature: dfdha4wzsh6mj9fgsqhm439agy6g7tsa X-Rspamd-Queue-Id: 8A1C2180083 Authentication-Results: imf16.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=GkHJIfSf; spf=none (imf16.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Server: rspam07 X-Rspam-User: X-HE-Tag: 1655151022-703569 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 13.06.22 21:43, Peter Xu wrote: > Hi, David, > > On Fri, Jun 10, 2022 at 11:42:06AM -0700, Nadav Amit wrote: >> On Jun 10, 2022, at 11:14 AM, David Hildenbrand wrote: >> >>> Similar to our MM_CP_DIRTY_ACCT handling for shared, writable mappings, we >>> can try mapping anonymous pages writable if they are exclusive, >>> the PTE is already dirty, and no special handling applies. Mapping the >>> PTE writable is essentially the same thing the write fault handler would do >>> in this case. >>> >>> Special handling is required for uffd-wp and softdirty tracking, so take >>> care of that properly. Also, leave PROT_NONE handling alone for now; >>> in the future, we could similarly extend the logic in do_numa_page() or >>> use pte_mk_savedwrite() here. Note that we'll now also check for uffd-wp in >>> case of VM_SHARED -- which is harmless and prepares for uffd-wp support for >>> shmem. >>> >>> While this improves mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE) >>> performance, it should also be a valuable optimization for uffd-wp, when >>> un-protecting. >>> >>> Applying the same logic to PMDs (anonymous THP, anonymous hugetlb) is >>> probably not worth the trouble, but could similarly be added if there is >>> demand. > > My memory was that Andrea had a version that used to have thp optimized > too. It'll be a slight pity to lose it if still possible, then we treat > thp and small pages the same logic and be all fair. Or is there any other > challenge that we're facing? Not really, but I assume performance gain will be minimal and might not be worth the trouble. I'm fairly busy (and not aware of Andreas version), so I can look at this, but it will be part of a separate patch because it will go on my TODO list. Not mad if someone beats me to it ;) > >>> >>> Results of a simple microbenchmark on my Ryzen 9 3900X, comparing the new >>> optimization (avoiding write faults) during mprotect() with softdirty >>> tracking, where we require a write fault. > > Are we comparing the mprotect() sequence operations against softdirty > clearing operation? Would it make more sense if we compare the same > mprotect() sequence to kernels that are before/after this patch applied? For simplicity I compared on the same kernel, one time exploting the optimization and one time disabling the optimization via softdirty. I can also simply measure without+with. Extra work for me to combine outputs :P > >>> >>> Running 1000 iterations each >>> >>> ========================================================== >>> Measuring memset() of 4096 bytes >>> First write access: >>> Min: 169 ns, Max: 8997 ns, Avg: 830 ns >>> Second write access: >>> Min: 80 ns, Max: 251 ns, Avg: 168 ns >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): >>> Min: 180 ns, Max: 290 ns, Avg: 190 ns >>> Write access after clearing softdirty: >>> Min: 451 ns, Max: 1774 ns, Avg: 470 ns >>> -> mprotect = 1.131 * second [avg] >>> -> mprotect = 0.404 * softdirty [avg] > > (I don't understand these two lines.. but maybe I'm the only one?) Most probably not. "mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE)" needs 113,1% the runtime compared with the "second write" access. "mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE)" needs 40% of the runtime compared with disabling the optimization via softdirty tracking. I may find time to clean that up a bit more to make it easier to consume for humans. > >>> ---------------------------------------------------------- >>> Measuring single byte access per page of 4096 bytes >>> First write access: >>> Min: 761 ns, Max: 1152 ns, Avg: 784 ns >>> Second write access: >>> Min: 130 ns, Max: 181 ns, Avg: 137 ns >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): >>> Min: 150 ns, Max: 1553 ns, Avg: 155 ns >>> Write access after clearing softdirty: >>> Min: 169 ns, Max: 1783 ns, Avg: 432 ns >>> -> mprotect = 1.131 * second [avg] >>> -> mprotect = 0.359 * softdirty [avg] >>> ========================================================== >>> Measuring memset() of 16384 bytes >>> First write access: >>> Min: 1594 ns, Max: 3497 ns, Avg: 2143 ns >>> Second write access: >>> Min: 250 ns, Max: 381 ns, Avg: 260 ns >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): >>> Min: 290 ns, Max: 1643 ns, Avg: 300 ns >>> Write access after clearing softdirty: >>> Min: 1242 ns, Max: 8987 ns, Avg: 1297 ns >>> -> mprotect = 1.154 * second [avg] >>> -> mprotect = 0.231 * softdirty [avg] >>> ---------------------------------------------------------- >>> Measuring single byte access per page of 16384 bytes >>> First write access: >>> Min: 1953 ns, Max: 2945 ns, Avg: 2008 ns >>> Second write access: >>> Min: 130 ns, Max: 912 ns, Avg: 142 ns >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): >>> Min: 160 ns, Max: 240 ns, Avg: 166 ns >>> Write access after clearing softdirty: >>> Min: 1112 ns, Max: 1513 ns, Avg: 1126 ns >>> -> mprotect = 1.169 * second [avg] >>> -> mprotect = 0.147 * softdirty [avg] >>> ========================================================== >>> Measuring memset() of 65536 bytes >>> First write access: >>> Min: 7524 ns, Max: 15650 ns, Avg: 7680 ns >>> Second write access: >>> Min: 251 ns, Max: 1323 ns, Avg: 648 ns >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): >>> Min: 270 ns, Max: 1282 ns, Avg: 736 ns >>> Write access after clearing softdirty: >>> Min: 4558 ns, Max: 12524 ns, Avg: 4623 ns >>> -> mprotect = 1.136 * second [avg] >>> -> mprotect = 0.159 * softdirty [avg] >>> ---------------------------------------------------------- >>> Measuring single byte access per page of 65536 bytes >>> First write access: >>> Min: 7083 ns, Max: 9027 ns, Avg: 7241 ns >>> Second write access: >>> Min: 140 ns, Max: 201 ns, Avg: 156 ns >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): >>> Min: 190 ns, Max: 451 ns, Avg: 197 ns >>> Write access after clearing softdirty: >>> Min: 3707 ns, Max: 5119 ns, Avg: 3958 ns >>> -> mprotect = 1.263 * second [avg] >>> -> mprotect = 0.050 * softdirty [avg] >>> ========================================================== >>> Measuring memset() of 524288 bytes >>> First write access: >>> Min: 58470 ns, Max: 87754 ns, Avg: 59353 ns >>> Second write access: >>> Min: 5180 ns, Max: 6863 ns, Avg: 5318 ns >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): >>> Min: 5871 ns, Max: 9358 ns, Avg: 6028 ns >>> Write access after clearing softdirty: >>> Min: 35797 ns, Max: 41338 ns, Avg: 36710 ns >>> -> mprotect = 1.134 * second [avg] >>> -> mprotect = 0.164 * softdirty [avg] >>> ---------------------------------------------------------- >>> Measuring single byte access per page of 524288 bytes >>> First write access: >>> Min: 53751 ns, Max: 59431 ns, Avg: 54506 ns >>> Second write access: >>> Min: 781 ns, Max: 2194 ns, Avg: 1123 ns >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): >>> Min: 161 ns, Max: 1282 ns, Avg: 622 ns >>> Write access after clearing softdirty: >>> Min: 30888 ns, Max: 34565 ns, Avg: 31229 ns >>> -> mprotect = 0.554 * second [avg] >>> -> mprotect = 0.020 * softdirty [avg] >>> >>> Cc: Linus Torvalds >>> Cc: Andrew Morton >>> Cc: Nadav Amit >>> Cc: Dave Hansen >>> Cc: Andrea Arcangeli >>> Cc: Peter Xu >>> Cc: Yang Shi >>> Cc: Hugh Dickins >>> Cc: Mel Gorman >>> Signed-off-by: David Hildenbrand >>> --- >>> >>> v1 -> v2: >>> * Rebased on v5.19-rc1 >>> * Rerun benchmark >>> * Fix minor spelling issues in subject+description >>> * Drop IS_ENABLED(CONFIG_MEM_SOFT_DIRTY) check >>> * Move pte_write() check into caller >>> >>> --- >>> mm/mprotect.c | 67 ++++++++++++++++++++++++++++++++++++++++++--------- >>> 1 file changed, 55 insertions(+), 12 deletions(-) >>> >>> diff --git a/mm/mprotect.c b/mm/mprotect.c >>> index ba5592655ee3..728772bf41c7 100644 >>> --- a/mm/mprotect.c >>> +++ b/mm/mprotect.c >>> @@ -38,6 +38,45 @@ >>> >>> #include "internal.h" >>> >>> +static inline bool can_change_pte_writable(struct vm_area_struct *vma, >>> + unsigned long addr, pte_t pte, >>> + unsigned long cp_flags) >>> +{ >>> + struct page *page; >>> + >>> + if ((vma->vm_flags & VM_SHARED) && !(cp_flags & MM_CP_DIRTY_ACCT)) >>> + /* >>> + * MM_CP_DIRTY_ACCT is only expressive for shared mappings; >>> + * without MM_CP_DIRTY_ACCT, there is nothing to do. >>> + */ >>> + return false; >>> + >>> + if (pte_protnone(pte) || !pte_dirty(pte)) >>> + return false; >>> + >>> + /* Do we need write faults for softdirty tracking? */ >>> + if ((vma->vm_flags & VM_SOFTDIRTY) && !pte_soft_dirty(pte)) >>> + return false; >>> + >>> + /* Do we need write faults for uffd-wp tracking? */ >>> + if (userfaultfd_pte_wp(vma, pte)) >>> + return false; >>> + >>> + if (!(vma->vm_flags & VM_SHARED)) { >>> + /* >>> + * We can only special-case on exclusive anonymous pages, >>> + * because we know that our write-fault handler similarly would >>> + * map them writable without any additional checks while holding >>> + * the PT lock. >>> + */ >>> + page = vm_normal_page(vma, addr, pte); >>> + if (!page || !PageAnon(page) || !PageAnonExclusive(page)) >>> + return false; >>> + } >>> + >>> + return true; >>> +} >>> + >> >> Looks good in general. Just wondering (out loud) whether it makes more sense >> to do all the vm_flags and cp_flags related checks in one of the callers >> (mprotect_fixup()?) and propagate whether to try to write-unprotect in >> cp_flags (e.g., by introducing new MM_CP_TRY_WRITE_UNPROTECT). > > I can see why David put it like that, because most of the checks are on > ptes not vm_flags. > > But I also agree on this point, especially if to put it in another way: > IMHO it'll be confusing if we keey MM_CP_DIRTY_ACCT==false for all private > pages even if we're going to take them into account and do smart unprotect > operations. > > So I'm wondering whether we could still at least move vm_flags check into > the mprotect_fixup() as suggested by Nadav, perhaps something like: > > diff --git a/mm/mprotect.c b/mm/mprotect.c > index ba5592655ee3..aefd5fe982af 100644 > --- a/mm/mprotect.c > +++ b/mm/mprotect.c > @@ -583,7 +583,11 @@ mprotect_fixup(struct mmu_gather *tlb, struct vm_area_struct *vma, > * held in write mode. > */ > vma->vm_flags = newflags; > - dirty_accountable = vma_wants_writenotify(vma, vma->vm_page_prot); > + if (vma->vm_flags & VM_SHARED) > + dirty_accountable = vma_wants_writenotify(vma, vma->vm_page_prot); > + else > + /* For private mappings, only if it's writable */ > + dirty_accountable = vma->vm_flags & VM_WRITE; > vma_set_page_prot(vma); > > change_protection(tlb, vma, start, end, vma->vm_page_prot, > > Then IIUC we could drop both the VM_WRITE check in change_pte_range(), and > also the VM_SHARED check above in can_change_pte_writable(). Not sure > whether that'll look slightly cleaner. I'll give it a shot and most probably rename dirty_accountable to something more expressive -- like Nadav proposed, for example. > > I'm also copying Peter Collingbourne because afaict he > proposed the initial idea (maybe worth some credit in the commit message?), Do you have a link to that conversation? Either my memory is messing with me or I did this without reading that mail (which I think, because it simply made sense with PageAnonExclusive at hand). Still, I can add a reference to that mail and mention that this was suggested earlier by Peter C.. Thanks! -- Thanks, David / dhildenb