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([2a01:599:915:8911:b13f:d972:e237:7fe2]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-441b8a286b9sm170753685e9.28.2025.05.06.07.34.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 06 May 2025 07:34:05 -0700 (PDT) Message-ID: Date: Tue, 6 May 2025 16:34:03 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 6/7] mm: Batch around can_change_pte_writable() To: Dev Jain , akpm@linux-foundation.org Cc: ryan.roberts@arm.com, willy@infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, catalin.marinas@arm.com, will@kernel.org, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, vbabka@suse.cz, jannh@google.com, anshuman.khandual@arm.com, peterx@redhat.com, joey.gouly@arm.com, ioworker0@gmail.com, baohua@kernel.org, kevin.brodsky@arm.com, quic_zhenhuah@quicinc.com, christophe.leroy@csgroup.eu, yangyicong@hisilicon.com, linux-arm-kernel@lists.infradead.org, namit@vmware.com, hughd@google.com, yang@os.amperecomputing.com, ziy@nvidia.com References: <20250429052336.18912-1-dev.jain@arm.com> <20250429052336.18912-7-dev.jain@arm.com> <25dcf969-e479-4d4a-a95c-0e83706af99e@redhat.com> <9a770aec-a020-4199-a53e-eddda657999d@redhat.com> <0857761c-bb4c-4ea5-a96a-90d555673527@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250506_073409_826294_025B9F94 X-CRM114-Status: GOOD ( 17.80 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 06.05.25 11:16, Dev Jain wrote: > > > On 29/04/25 2:57 pm, David Hildenbrand wrote: >> On 29.04.25 11:19, David Hildenbrand wrote: >>> >>>>    #include "internal.h" >>>> -bool can_change_pte_writable(struct vm_area_struct *vma, unsigned >>>> long addr, >>>> -                 pte_t pte) >>>> +bool can_change_ptes_writable(struct vm_area_struct *vma, unsigned >>>> long addr, >>>> +                  pte_t pte, struct folio *folio, unsigned int nr) >>>>    { >>>>        struct page *page; >>>> @@ -67,8 +67,9 @@ bool can_change_pte_writable(struct vm_area_struct >>>> *vma, unsigned long addr, >>>>             * write-fault handler similarly would map them writable >>>> without >>>>             * any additional checks while holding the PT lock. >>>>             */ >>>> -        page = vm_normal_page(vma, addr, pte); >>>> -        return page && PageAnon(page) && PageAnonExclusive(page); >>>> +        if (!folio) >>>> +            folio = vm_normal_folio(vma, addr, pte); >>>> +        return folio_test_anon(folio) && ! >>>> folio_maybe_mapped_shared(folio); >>> >>> Oh no, now I spot it. That is horribly wrong. >>> >>> Please understand first what you are doing. >> >> Also, would expect that the cow.c selftest would catch that: >> >> "vmsplice() + unmap in child with mprotect() optimization" >> >> After fork() we have a R/O PTE in the parent. Our child then uses >> vmsplice() and unmaps the R/O PTE, meaning it is only left mapped by the >> parent. >> >> ret = mprotect(mem, size, PROT_READ); >> ret |= mprotect(mem, size, PROT_READ|PROT_WRITE); >> >> should turn the PTE writable, although it shouldn't. >> >> If that test case does not detect the issue you're introducing, we >> should look into adding a test case that detects it. >> > > Hi David, I am afraid I don't understand my mistake :( PageAnon(page) > boils down to folio_test_anon(folio). Next we want to determine whether > the page underlying a PTE is mapped exclusively or not. No. :) There is your mistake. We need to know if this folio page is *exclusive* not if the folio is *mapped exclusively*. I know, it's confusing, but that's an important distinction. You really have to test all PAE bits. I recently sent a mail on how we could remove PAE and encode it in the PTE value itself (W || (!W +dirty)), which would mean that we could batch more easily. For the time being, we have to stick with what we have. -- Cheers, David / dhildenb