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[2003:cb:c706:e00:8d96:5dba:6bc4:6e89]) by smtp.gmail.com with ESMTPSA id bg42-20020a05600c3caa00b003a31b79dc0esm12139062wmb.1.2022.07.20.11.00.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 Jul 2022 11:00:20 -0700 (PDT) Message-ID: <97e82e31-2411-501e-d401-d175eb24642f@redhat.com> Date: Wed, 20 Jul 2022 20:00:19 +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 Cc: Linux MM , LKML , Andrew Morton , Mike Rapoport , Axel Rasmussen , 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> <09d84297-65d5-a3df-fdc0-a7168cdb0798@redhat.com> <99AFB0A0-B2F8-420A-8164-1705EC14B255@vmware.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <99AFB0A0-B2F8-420A-8164-1705EC14B255@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: 8bit ARC-Authentication-Results: i=1; imf05.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=HCaeGCph; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf05.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.129.124) smtp.mailfrom=david@redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1658340024; a=rsa-sha256; cv=none; b=h6PJbq6BTf8ACS6zZKevVLEy0E/BrcxDu2Qx6nRrmhpiY5cl08r7btcZT/LvWgi230P80v eja5IDXln/cC+09xOjwxBefhwnQoYCWL/WGezw+PcpYyyvN65RvCS0UsFzaRythGxoIbBh 5NU2VypJqQ3cZEq4w81cd2qJKxTAXXE= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1658340024; 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=daD9q7+pH5XOO19X8ngAWhNCL1CJM0I+kmMSrlE+EFk=; b=fvAvDC9+ma6GS4GNUXlWuYFFgpWfH16wivyvMjQn0/+2HJF0dLuQ3ZOCRrbInsoHR8ye4v T/8ovPhmidqQ/netQ3zB5qudmp9GkvwZ/nGIRmeV7jcCKOtisTblmPOyJuZuhKTqicjJZF Ggkt9L0LSqyDimP9FC8k2VPpbLbYTJo= X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: D0AE3100078 Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=HCaeGCph; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf05.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.129.124) smtp.mailfrom=david@redhat.com X-Stat-Signature: ewoqmh37q7sn3esuahrradxippt1wkxs X-HE-Tag: 1658340023-395776 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 20.07.22 19:36, Nadav Amit wrote: > On Jul 20, 2022, at 2:42 AM, David Hildenbrand wrote: > >> ⚠ External Email >> >> 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. > > We check !pte_write(). What am I missing in your question? My patch review skills have seen better days. I thought you'd be removing the pte_write() check ... :( Tired eyes ... > > Having said that, I do notice now that pte_mkdirty() should not be done > only this condition is fulfilled. Instead we should just have > something like: > > if (will_need) { > ptent = pte_mkyoung(ptent); > if (pte_write(ptent)) > ptent = pte_mkdirty(ptent); > } As can_change_pte_writable() will fail if it stumbles over a !pte_dirty page in current code ... so I assume you would have that code before the actual pte_mkwrite() logic, correct? -- Thanks, David / dhildenb