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[2003:d8:2f4e:3200:c99d:a38b:3f3a:d4b3]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-429898acc63sm3522878f8f.27.2025.10.23.04.49.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 23 Oct 2025 04:49:23 -0700 (PDT) Message-ID: Date: Thu, 23 Oct 2025 13:49:22 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/filemap: Implement fast short reads 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> <56d9f1d9-fc20-4be8-b64a-07beac3c64d0@redhat.com> <5b33b587-ffd1-4a25-95e5-5f803a935a57@redhat.com> <7fmiqrcyiccff5okrs7sdz3i63mp376f2r76e4r5c2miluwk76@567sm46qop5h> 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: <7fmiqrcyiccff5okrs7sdz3i63mp376f2r76e4r5c2miluwk76@567sm46qop5h> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 23.10.25 13:40, Kiryl Shutsemau wrote: > On Thu, Oct 23, 2025 at 01:11:43PM +0200, David Hildenbrand wrote: >> On 23.10.25 13:10, David Hildenbrand wrote: >>> 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 >> >> Sorry I meant here "while that works for load_unaligned_zeropad()". > > Do we have other random reads? > > For unaccepted memory, we care about touching memory that was never > allocated because accepting memory is one way road. Right, but I suspect if you get a random read (as the unaccepted memory doc states) you'd be in trouble as well. The "nice" thing about unaccepted memory is that it's a one way road indeed, and at some point the system will not have unaccepted memory anymore. > > I only know about load_unaligned_zeropad() that does reads like this. Do > you know others? No, I am not aware of others. Most code that could read random memory (kcore, vmcore) was fixed to exclude pages we know are unsafe to touch. Code where might speculatively access the "struct page" after it might already have been freed (speculative pagecache lookups, GUP-fast) will just back off and never read page content. We avoid such random memory reads as best we can, as it's just a pain to deal with (like load_unaligned_zeropad(), which i would just wish we could get rid of now that it's present again in my memory. :( ). -- Cheers David / dhildenb