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[2003:cb:c706:e00:8d96:5dba:6bc4:6e89]) by smtp.gmail.com with ESMTPSA id a6-20020a05600c068600b003a2e5caab12sm2576603wmn.47.2022.07.20.08.19.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 Jul 2022 08:19:59 -0700 (PDT) Message-ID: <23a9d678-487e-5940-4cde-dc53d920fb48@redhat.com> Date: Wed, 20 Jul 2022 17:19:58 +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 03/14] mm/mprotect: allow exclusive anon pages to be writable 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> <20220718120212.3180-4-namit@vmware.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220718120212.3180-4-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-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1658330409; a=rsa-sha256; cv=none; b=LSMPUv9JtiZ/IhglWdYYcZfQf4drPSApEKj/5q6Rn3/NRkqtbkWI7hpRA12+jcELBt4lZl vzcCdXdzmhpdCA5Xf3MH/W1wVoTFDcBLJUUthMpqxeKbl6pi9n4rkFDwWJcZ02qS1LLBUm /gv3XnSa63I4SBW/qL0DTFf1gbsWqhs= ARC-Authentication-Results: i=1; imf25.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="Pccbj9/z"; spf=none (imf25.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=1658330409; 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=vXIv8z6wgYrxtRmSkO6MksNcPo7L1KXy6iCAgLmfCq8=; b=AxxrWDdMpsk31fgPRjT+uhEYoIqfL1W/gmmY4rpJ3/GcqEz878Ko+v/md1sRlljJvdDRif tpbNrqTC1/LOV12XGpgiAO6Q662urFJ9PJKmBfYsyp1JiZ05GwLM/sKM60YMlQmcOBsNX4 9v5VYWeLgSYj6yl65rkuJ7OojaGxMPE= X-Rspamd-Queue-Id: DC03EA009A Authentication-Results: imf25.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="Pccbj9/z"; spf=none (imf25.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-Rspam-User: X-Rspamd-Server: rspam06 X-Stat-Signature: jaiacppzyho5gosykr95op411khuf6e9 X-HE-Tag: 1658330408-771813 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 14:02, Nadav Amit wrote: > From: Nadav Amit > > Anonymous pages might have the dirty bit clear, but this should not > prevent mprotect from making them writable if they are exclusive. > Therefore, skip the test whether the page is dirty in this case. > > 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 > Signed-off-by: Nadav Amit > --- > mm/mprotect.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/mm/mprotect.c b/mm/mprotect.c > index 34c2dfb68c42..da5b9bf8204f 100644 > --- a/mm/mprotect.c > +++ b/mm/mprotect.c > @@ -45,7 +45,7 @@ static inline bool can_change_pte_writable(struct vm_area_struct *vma, > > VM_BUG_ON(!(vma->vm_flags & VM_WRITE) || pte_write(pte)); > > - if (pte_protnone(pte) || !pte_dirty(pte)) > + if (pte_protnone(pte)) > return false; > > /* Do we need write faults for softdirty tracking? */ > @@ -66,7 +66,8 @@ static inline bool can_change_pte_writable(struct vm_area_struct *vma, > page = vm_normal_page(vma, addr, pte); > if (!page || !PageAnon(page) || !PageAnonExclusive(page)) > return false; > - } > + } else if (!pte_dirty(pte)) > + return false; > > return true; > } When I wrote that code, I was wondering how often that would actually happen in practice -- and if we care about optimizing that. Do you have a gut feeling in which scenarios this would happen and if we care? If the page is in the swapcache and was swapped out, you'd be requiring a writeback even though nobody modified the page and possibly isn't going to do so in the near future. -- Thanks, David / dhildenb