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[2003:cb:c70f:2b00:f928:47d8:c9be:9d87]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42ac517a65dsm17825185e9.34.2024.08.22.02.29.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 22 Aug 2024 02:29:53 -0700 (PDT) Message-ID: <04dccff9-87dd-4e97-a712-b62fa51b32f7@redhat.com> Date: Thu, 22 Aug 2024 11:29:51 +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> 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 21.08.24 12:03, Qi Zheng wrote: > > > On 2024/8/21 17:53, David Hildenbrand wrote: >> On 21.08.24 11:51, Qi Zheng wrote: >>> >>> >>> On 2024/8/21 17:41, David Hildenbrand wrote: >>>> On 21.08.24 11:24, Qi Zheng wrote: >>>>> >>>>> >>>>> On 2024/8/21 17:17, LEROY Christophe wrote: >>>>>> >>>>>> >>>>>> Le 21/08/2024 à 10:18, Qi Zheng a écrit : >>>>>>> In handle_pte_fault(), we may modify the vmf->pte after acquiring the >>>>>>> vmf->ptl, so convert it to using pte_offset_map_maywrite_nolock(). >>>>>>> But >>>>>>> since we already do the pte_same() check, so there is no need to get >>>>>>> pmdval to do pmd_same() check, just pass NULL to pmdvalp parameter. >>>>>>> >>>>>>> Signed-off-by: Qi Zheng >>>>>>> --- >>>>>>>      mm/memory.c | 9 +++++++-- >>>>>>>      1 file changed, 7 insertions(+), 2 deletions(-) >>>>>>> >>>>>>> diff --git a/mm/memory.c b/mm/memory.c >>>>>>> index 93c0c25433d02..d3378e98faf13 100644 >>>>>>> --- a/mm/memory.c >>>>>>> +++ b/mm/memory.c >>>>>>> @@ -5504,9 +5504,14 @@ static vm_fault_t handle_pte_fault(struct >>>>>>> vm_fault *vmf) >>>>>>>               * pmd by anon khugepaged, since that takes mmap_lock in >>>>>>> write >>>>>>>               * mode; but shmem or file collapse to THP could still >>>>>>> morph >>>>>>>               * it into a huge pmd: just retry later if so. >>>>>>> +         * >>>>>>> +         * Use the maywrite version to indicate that vmf->pte >>>>>>> will be >>>>>>> +         * modified, but since we will use pte_same() to detect the >>>>>>> +         * change of the pte entry, there is no need to get pmdval. >>>>>>>               */ >>>>>>> -        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? > > 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. -- Cheers, David / dhildenb