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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 Content-Language: en-US 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> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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