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[2003:cb:c706:e00:8d96:5dba:6bc4:6e89]) by smtp.gmail.com with ESMTPSA id h12-20020a05600c28cc00b003a325bd8517sm2190898wmd.5.2022.07.20.02.42.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 Jul 2022 02:42:56 -0700 (PDT) Message-ID: <09d84297-65d5-a3df-fdc0-a7168cdb0798@redhat.com> Date: Wed, 20 Jul 2022 11:42:50 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [RFC PATCH 01/14] userfaultfd: set dirty and young on writeprotect To: Nadav Amit , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Andrew Morton , Mike Rapoport , Axel Rasmussen , Nadav Amit , Andrea Arcangeli , Andrew Cooper , Andy Lutomirski , Dave Hansen , Peter Xu , Peter Zijlstra , Thomas Gleixner , Will Deacon , Yu Zhao , Nick Piggin References: <20220718120212.3180-1-namit@vmware.com> <20220718120212.3180-2-namit@vmware.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220718120212.3180-2-namit@vmware.com> 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-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1658310181; 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=6RKfphNn3mPOW1MXkGNfh4n+QtAmR71ynMhMeD/fO8E=; b=YQTRDpp/xfcjMXxtN1Fmw6iPaeSySCUaln67Q/IdVgfuYG5/jaTkPcB7Jtk4WUKI+5UuG8 2qVwKUuahlSCd+Cs2XbEU1w3BHIPCLFtPt6qJhXHjxz5W3GlhNgfvgThthXoV/xUCpsm2H KtZ2hgDip44TQpEm/Xf0CYl9Ijio0Co= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1658310181; a=rsa-sha256; cv=none; b=UZt+wcHjPGktOahFcRblu7KR3ac9zsvOKAwlufaKp4yOYkdfvuQmk8g72RqX7rA6uBS38E eyDZUDSRyizN7L3/Cn0Z9f82BUjrL62r/1YmO0P8vUSb+P/EvjjrxZhTkF8MmkJMxxJVLS RFo1dvFpTD4Rk10Vw6xGKqRSIRICEtY= ARC-Authentication-Results: i=1; imf14.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=KvPxQXWf; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf14.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.129.124) smtp.mailfrom=david@redhat.com X-Rspamd-Queue-Id: 53138100014 Authentication-Results: imf14.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=KvPxQXWf; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf14.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.129.124) smtp.mailfrom=david@redhat.com X-Rspamd-Server: rspam12 X-Rspam-User: X-Stat-Signature: n19r6msrfqzqwu1kjxnti1w8so6s3fuf X-HE-Tag: 1658310181-538737 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 18.07.22 14:01, Nadav Amit wrote: > From: Nadav Amit > > When userfaultfd makes a PTE writable, it can now change the PTE > directly, in some cases, without going triggering a page-fault first. > Yet, doing so might leave the PTE that was write-unprotected as old and > clean. At least on x86, this would cause a >500 cycles overhead when the > PTE is first accessed. > > Use MM_CP_WILL_NEED to set the PTE as young and dirty when userfaultfd > gets a hint that the page is likely to be used. Avoid changing the PTE > to young and dirty in other cases to avoid excessive writeback and > messing with the page reclamation logic. > > Cc: Andrea Arcangeli > Cc: Andrew Cooper > Cc: Andrew Morton > Cc: Andy Lutomirski > Cc: Dave Hansen > Cc: David Hildenbrand > Cc: Peter Xu > Cc: Peter Zijlstra > Cc: Thomas Gleixner > Cc: Will Deacon > Cc: Yu Zhao > Cc: Nick Piggin > --- > include/linux/mm.h | 2 ++ > mm/mprotect.c | 9 ++++++++- > mm/userfaultfd.c | 8 ++++++-- > 3 files changed, 16 insertions(+), 3 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 9cc02a7e503b..4afd75ce5875 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -1988,6 +1988,8 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma, > /* Whether this change is for write protecting */ > #define MM_CP_UFFD_WP (1UL << 2) /* do wp */ > #define MM_CP_UFFD_WP_RESOLVE (1UL << 3) /* Resolve wp */ > +/* Whether to try to mark entries as dirty as they are to be written */ > +#define MM_CP_WILL_NEED (1UL << 4) > #define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \ > MM_CP_UFFD_WP_RESOLVE) > > diff --git a/mm/mprotect.c b/mm/mprotect.c > index 996a97e213ad..34c2dfb68c42 100644 > --- a/mm/mprotect.c > +++ b/mm/mprotect.c > @@ -82,6 +82,7 @@ static unsigned long change_pte_range(struct mmu_gather *tlb, > bool prot_numa = cp_flags & MM_CP_PROT_NUMA; > bool uffd_wp = cp_flags & MM_CP_UFFD_WP; > bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE; > + bool will_need = cp_flags & MM_CP_WILL_NEED; > > tlb_change_page_size(tlb, PAGE_SIZE); > > @@ -172,6 +173,9 @@ static unsigned long change_pte_range(struct mmu_gather *tlb, > ptent = pte_clear_uffd_wp(ptent); > } > > + if (will_need) > + ptent = pte_mkyoung(ptent); > + > /* > * In some writable, shared mappings, we might want > * to catch actual write access -- see > @@ -187,8 +191,11 @@ static unsigned long change_pte_range(struct mmu_gather *tlb, > */ > if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) && > !pte_write(ptent) && Why would we want to check if we can set something writable if it already *is* writable? That doesn't make sense to me. > - can_change_pte_writable(vma, addr, ptent)) > + can_change_pte_writable(vma, addr, ptent)) { > ptent = pte_mkwrite(ptent); > + if (will_need) > + ptent = pte_mkdirty(ptent); > + } -- Thanks, David / dhildenb