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[2003:cb:c70f:2b00:f928:47d8:c9be:9d87]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42ac515a2a5sm23144385e9.20.2024.08.22.05.19.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 22 Aug 2024 05:19:02 -0700 (PDT) Message-ID: <38d6caeb-1aba-43d2-8daf-12b9aaba77bd@redhat.com> Date: Thu, 22 Aug 2024 14:19:01 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 06/14] mm: handle_pte_fault() use pte_offset_map_maywrite_nolock() To: Qi Zheng Cc: LEROY Christophe , "hughd@google.com" , "willy@infradead.org" , "muchun.song@linux.dev" , "vbabka@kernel.org" , "akpm@linux-foundation.org" , "rppt@kernel.org" , "vishal.moola@gmail.com" , "peterx@redhat.com" , "ryan.roberts@arm.com" , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "linux-arm-kernel@lists.infradead.org" , "linuxppc-dev@lists.ozlabs.org" References: <239432a0bc56464e58a6baf3622fdc72526c8d57.1724226076.git.zhengqi.arch@bytedance.com> <6a586524-5116-4eaf-b4f3-c1aea290d7c1@cs-soprasteria.com> <4b867535-8481-4fa1-bed1-ad25a76682f0@redhat.com> <61c05197-0baa-4680-ad24-5965ba37dc35@redhat.com> <83142dc8-edcf-4e47-8215-8b359a2b7156@bytedance.com> <04dccff9-87dd-4e97-a712-b62fa51b32f7@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 22.08.24 14:17, Qi Zheng wrote: > Hi David, > > On 2024/8/22 17:29, David Hildenbrand wrote: >> On 21.08.24 12:03, Qi Zheng wrote: >>> >>> > > [...] > >>>>>>>>> -        vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, >>>>>>>>> vmf->pmd, >>>>>>>>> -                         vmf->address, &vmf->ptl); >>>>>>>>> +        vmf->pte = pte_offset_map_maywrite_nolock(vmf->vma->vm_mm, >>>>>>>>> +                              vmf->pmd, vmf->address, >>>>>>>>> +                              NULL, &vmf->ptl); >>>>>> >>>>>> I think we discussed that passing NULL should be forbidden for that >>>>>> function. >>>>> >>>>> Yes, but for some maywrite case, there is no need to get pmdval to >>>>> do pmd_same() check. So I passed NULL and added a comment to >>>>> explain this. >>>> >>>> I wonder if it's better to pass a dummy variable instead. One has to >>>> think harder why that is required compared to blindly passing "NULL" :) >>> >>> You are afraid that subsequent caller will abuse this function, right? >> >> Yes! "oh, I don't need a pmdval, why would I? let's just pass NULL, >> easy" :) >> >>> My initial concern was that this would add a useless local vaiable, but >>> perhaps that is not a big deal. >> >> How many of these "special" instances do we have? > > We have 5 such special instances. > >> >>> >>> Both are fine for me. ;) >> >> Also no strong opinion, but having to pass a variable makes you think >> what you are supposed to do with it and why it is not optional. > > Yeah, I added 'BUG_ON(!pmdvalp);' in pte_offset_map_ro_nolock(), and > have updated the v2 version [1]. No BUG_ON please :) VM_WARN_ON_ONCE() is good enough. -- Cheers, David / dhildenb