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[2003:d8:2f09:9c00:8173:2a94:640d:dd31]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b7e7f14bbsm237407345e9.8.2025.09.03.02.33.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 03 Sep 2025 02:34:00 -0700 (PDT) Message-ID: Date: Wed, 3 Sep 2025 11:33:59 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Where to put page->memdesc initially To: Matthew Wilcox Cc: linux-mm@kvack.org, Jason Gunthorpe References: From: David Hildenbrand 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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 5DdkT2xEVgAxsxeB3buXRJTqb4jF3ZG_lZVleyRbEoc_1756892042 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: A30C6C0015 X-Stat-Signature: xny5r73ekhejcydmjjdhiqmxunwiwe5j X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1756892044-677100 X-HE-Meta: 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 gYDZ0uXc 50L5e/tGnRWU51Xd178tmWzurVEiEAWKn21FCj0XbQuo25r+TtK7YXHFvKQrUEgPemV5YGkHxPnsL//97R3h62YfhKJd1krJo378teFPPh2raEf4aw6/hdN0WLxVy0+KUHwoELEKLhMCUnebFBQ6maG9rGNB0Dw9G7mUNQtzeqZiWQidsw48/Ag4CKJRmjS/1V/3e97apktGoyt2gWoEoxAeaIRGgyYrwi/JTe1SZU9Jvocl/bSGzhgqt0p0fA/qd0CqcZot3p2HjJWNo4RVEsXsvvHSkn9eAovjW9xmWRpTIZpmM+6YtNj6HAiRrFDi/dyechdQlk0wO3/3eTCV7R1sWHQbKsX0AGoUFAlyfuKkxMIvyAm9CEArxkBjHZ4pAW/6S8ulLnDvgaVdx2r8q7GsKZ9FKrkwmZwKlKqjF+JTEJ5M= 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 02.09.25 23:06, Matthew Wilcox wrote: > On Tue, Sep 02, 2025 at 10:09:49PM +0200, David Hildenbrand wrote: >> Would you want to move the page type already from the mapcount into the >> memdesc? That sounds challenging, because for any typed folios we would >> not be allowed to reuse a field we want to use for the memdesc. IIRC< >> hugetlb pretty much uses all of it. > > That would definitely be part of the same series. But possibly not the > same patch. I think the series has to include separate allocations for > slab, folio and whichever other memdescs won't fit into 32 bytes. I was wondering whether there could be a single patch where we do this change (separate allocations), and just prepare the code in previous patches for that accordingly, such that the resulting patch is still reasonable small. I feel like this way of splitting patches might cause unnecessary headaches :) > >> The easy way out for now would be making this page type specific: Only >> selected typed pages will store the memdesc (here: slab pointer) e.g., >> in the old page->mapping place. >> >> So PageSlab() still checks the existing page type, put page_slab() would >> simply lookup the pointer in the old page->mapping place. > > I *think* that's roughly the same as what I'm proposing, except > that we already have a meaning for "the bottom two bits of > folio->mapping are set", so there's potential confusion for > folio_test_anon() & friends. IIRC, we must always make sure to never call folio_test_anon() on something that is a slab already. But if in doubt, we could use bit[2] in ->mapping, which should still be unussed IIRC. >> >>> There are a few overlapping uses of these bits in struct page, so if we do >>> nothing we may get confused. We can deal with mlock_count and order (for >>> pcp_llist). But the biggest problem is the first tail page of a folio. >>> Depending on word size and endianness, there are four different atomic_t >>> fields that overlap with page->lru.prev. That can't be solved by using >>> a different field in struct page; the first tail page is jam-packed. >>> >>> So, page_slab() will first load page->memdesc (the same bits as >>> page->lru.prev), check the bottom four bits match the slab memdesc, and >>> also check page->page_type matches PGTY_slab. I don't like this a lot, >>> because it's two loads rather than one atomic load, but it should only >>> be present for one commit. >> >> As a first step, I would really not use the bottom four bits. Why >> perform two type checks initially? > > I'm concerned by things like compaction that are executing > asynchronously and might see a page mid-transition. Or something like > GUP or lockless pagecache lookup that might get a stale page pointer. > It's a lot easier to reason about if we can do a single load and treat > that as a source of truth (with the appropriate reloads to make sure > nothing changed after we got a refcount). Doing two loads makes > my brain hurt a bit because it introduces more possibilities for > inconsistency. I'll need to write it up pretty carefully (which > annoys me because we're going to need it for a single or very > few commits ...) Makes sense, but maybe we can avoid all that by just structuring the patches differently :) -- Cheers David / dhildenb