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[2003:d8:2f4e:3200:c99d:a38b:3f3a:d4b3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-475c43900e1sm106702345e9.17.2025.10.23.04.10.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 23 Oct 2025 04:10:54 -0700 (PDT) Message-ID: <56d9f1d9-fc20-4be8-b64a-07beac3c64d0@redhat.com> Date: Thu, 23 Oct 2025 13:10:53 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/filemap: Implement fast short reads From: David Hildenbrand To: Kiryl Shutsemau Cc: Andrew Morton , Matthew Wilcox , Linus Torvalds , Alexander Viro , Christian Brauner , Jan Kara , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org References: <20251017141536.577466-1-kirill@shutemov.name> <06333766-fb79-4deb-9b53-5d1230b9d88d@redhat.com> 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: <06333766-fb79-4deb-9b53-5d1230b9d88d@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: NHOUAHYklUMstEB3yqDlwdHKWWpYb_oWAsyJhlHkK4U_1761217856 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: gox1f5cgbcih3syzp33igej7uksh8ch7 X-Rspamd-Queue-Id: 18EACA0006 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1761217858-870131 X-HE-Meta: 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 L7aEzpWJ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 23.10.25 12:54, David Hildenbrand wrote: > On 23.10.25 12:31, Kiryl Shutsemau wrote: >> On Wed, Oct 22, 2025 at 07:28:27PM +0200, David Hildenbrand wrote: >>> "garbage" as in pointing at something without a direct map, something that's >>> protected differently (MTE? weird CoCo protection?) or even worse MMIO with >>> undesired read-effects. >> >> Pedro already points to the problem with missing direct mapping. >> _nofault() copy should help with this. > > Yeah, we do something similar when reading the kcore for that reason. > >> >> Can direct mapping ever be converted to MMIO? It can be converted to DMA >> buffer (which is fine), but MMIO? I have not seen it even in virtualized >> environments. > > I recall discussions in the context of PAT and the adjustment of caching > attributes of the direct map for MMIO purposes: so I suspect there are > ways that can happen, but I am not 100% sure. > > > Thinking about it, in VMs we have the direct map set on balloon inflated > pages that should not be touched, not even read, otherwise your > hypervisor might get very angry. That case we could likely handle by > checking whether the source page actually exists and doesn't have > PageOffline() set, before accessing it. A bit nasty. > > A more obscure cases would probably be reading a page that was poisoned > by hardware and is not expected to be used anymore. Could also be > checked by checking the page. > > Essentially all cases where we try to avoid reading ordinary memory > already when creating memory dumps that might have a direct map. > > > Regarding MTE and load_unaligned_zeropad(): I don't know unfortunately. Looking into this, I'd assume the exception handler will take care of it. load_unaligned_zeropad() is interesting if there is a direct map but the memory should not be touched (especially regarding PageOffline and memory errors). I read drivers/firmware/efi/unaccepted_memory.c where we there is a lengthy discussion about guard pages and how that works for unaccepted memory. While it works for unaccepted memory, it wouldn't work for other random accesses as I suspect we could produce in this patch. -- Cheers David / dhildenb