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[2003:d8:2f1f:3f00:731a:f5e5:774e:d40c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b7e7d2393sm190896135e9.3.2025.09.02.02.55.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 Sep 2025 02:55:01 -0700 (PDT) Message-ID: <8f571dc9-f2d4-46c2-a4b4-6854fa31da2e@redhat.com> Date: Tue, 2 Sep 2025 11:54:59 +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 v5 03/12] mm: introduce AS_NO_DIRECT_MAP To: Fuad Tabba , "Roy, Patrick" Cc: "ackerleytng@google.com" , "Manwaring, Derek" , "Thomson, Jack" , "Kalyazin, Nikita" , "kvm@vger.kernel.org" , "kvmarm@lists.linux.dev" , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "pbonzini@redhat.com" , "rppt@kernel.org" , "seanjc@google.com" , "vbabka@suse.cz" , "will@kernel.org" , "Cali, Marco" References: <20250902091810.4854-1-roypat@amazon.co.uk> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 02.09.25 11:21, Fuad Tabba wrote: > On Tue, 2 Sept 2025 at 10:18, Roy, Patrick wrote: >> >> On Tue, 2025-09-02 at 09:50 +0100, Fuad Tabba wrote: >>> On Tue, 2 Sept 2025 at 09:46, David Hildenbrand wrote: >>>> >>>> On 02.09.25 09:59, Fuad Tabba wrote: >>>>> Hi Patrick, >>>>> >>>>> On Mon, 1 Sept 2025 at 15:56, Roy, Patrick wrote: >>>>>> >>>>>> On Mon, 2025-09-01 at 14:54 +0100, "Roy, Patrick" wrote: >>>>>>> >>>>>>> Hi Fuad! >>>>>>> >>>>>>> On Thu, 2025-08-28 at 11:21 +0100, Fuad Tabba wrote: >>>>>>>> Hi Patrick, >>>>>>>> >>>>>>>> On Thu, 28 Aug 2025 at 10:39, Roy, Patrick wrote: >>>>>>>>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h >>>>>>>>> index 12a12dae727d..b52b28ae4636 100644 >>>>>>>>> --- a/include/linux/pagemap.h >>>>>>>>> +++ b/include/linux/pagemap.h >>>>>>>>> @@ -211,6 +211,7 @@ enum mapping_flags { >>>>>>>>> folio contents */ >>>>>>>>> AS_INACCESSIBLE = 8, /* Do not attempt direct R/W access to the mapping */ >>>>>>>>> AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9, >>>>>>>>> + AS_NO_DIRECT_MAP = 10, /* Folios in the mapping are not in the direct map */ >>>>>>>>> /* Bits 16-25 are used for FOLIO_ORDER */ >>>>>>>>> AS_FOLIO_ORDER_BITS = 5, >>>>>>>>> AS_FOLIO_ORDER_MIN = 16, >>>>>>>>> @@ -346,6 +347,21 @@ static inline bool mapping_writeback_may_deadlock_on_reclaim(struct address_spac >>>>>>>>> return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags); >>>>>>>>> } >>>>>>>>> >>>>>>>>> +static inline void mapping_set_no_direct_map(struct address_space *mapping) >>>>>>>>> +{ >>>>>>>>> + set_bit(AS_NO_DIRECT_MAP, &mapping->flags); >>>>>>>>> +} >>>>>>>>> + >>>>>>>>> +static inline bool mapping_no_direct_map(struct address_space *mapping) >>>>>>>>> +{ >>>>>>>>> + return test_bit(AS_NO_DIRECT_MAP, &mapping->flags); >>>>>>>>> +} >>>>>>>>> + >>>>>>>>> +static inline bool vma_is_no_direct_map(const struct vm_area_struct *vma) >>>>>>>>> +{ >>>>>>>>> + return vma->vm_file && mapping_no_direct_map(vma->vm_file->f_mapping); >>>>>>>>> +} >>>>>>>>> + >>>>>>>> Any reason vma is const whereas mapping in the function that it calls >>>>>>>> (defined above it) isn't? >>>>>>> >>>>>>> Ah, I cannot say that that was a conscious decision, but rather an artifact of >>>>>>> the code that I looked at for reference when writing these two simply did it >>>>>>> this way. Are you saying both should be const, or neither (in my mind, both >>>>>>> could be const, but the mapping_*() family of functions further up in this file >>>>>>> dont take const arguments, so I'm a bit unsure now)? >>>>>> >>>>>> Hah, just saw >>>>>> https://lore.kernel.org/linux-mm/20250901123028.3383461-3-max.kellermann@ionos.com/. >>>>>> Guess that means "both should be const" then :D >>>>> >>>>> I don't have any strong preference regarding which way, as long as >>>>> it's consistent. The thing that should be avoided is having one >>>>> function with a parameter marked as const, pass that parameter (or >>>>> something derived from it), to a non-const function. >>>> >>>> I think the compiler will tell you that that is not ok (and you'd have >>>> to force-cast the const it away). >>> >>> Not for the scenario I'm worried about. The compiler didn't complain >>> about this (from this patch): >>> >>> +static inline bool mapping_no_direct_map(struct address_space *mapping) >>> +{ >>> + return test_bit(AS_NO_DIRECT_MAP, &mapping->flags); >>> +} >>> + >>> +static inline bool vma_is_no_direct_map(const struct vm_area_struct *vma) >>> +{ >>> + return vma->vm_file && mapping_no_direct_map(vma->vm_file->f_mapping); >>> +} >>> >>> vma_is_no_direct_map() takes a const, but mapping_no_direct_map() >>> doesn't. For now, mapping_no_direct_map() doesn't modify anything. But >>> it could, and the compiler wouldn't complain. >> >> Wouldn't this only be a problem if vma->vm_file->f_mapping was a 'const struct >> address_space *const'? I thought const-ness doesn't leak into pointers (e.g. >> even above, vma_is_no_direct_map isn't allowed to make vma point at something >> else, but it could modify the pointed-to vm_area_struct). > > That's the thing, constness checks don't carry over to pointers within > a struct, but a person reading the code would assume that a function > with a parameter marked as const wouldn't modify anything related to > that parameter. Ah, thanks, I forgot that detail, it's only for embedded structs but not pointers. I wonder if something (sparse?) could detect such cases. -- Cheers David / dhildenb