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[2003:d8:2f1f:3f00:731a:f5e5:774e:d40c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b66f2041fsm173854155e9.5.2025.09.02.13.09.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 Sep 2025 13:09:49 -0700 (PDT) Message-ID: Date: Tue, 2 Sep 2025 22:09:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Where to put page->memdesc initially To: Matthew Wilcox , linux-mm@kvack.org Cc: 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: 6OUm-1grwnLXkZdDchYooUxhS5ie3O5vGRLSTY8s02w_1756843791 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 91B0920011 X-Stat-Signature: 8pe5p4b3h5us15w7qt7zw3u71u1qe3su X-Rspam-User: X-HE-Tag: 1756843794-387763 X-HE-Meta: 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 XOVyJObr 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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 02.09.25 21:03, Matthew Wilcox wrote: > With the recent patches to slab, I'm just about ready to allocate struct > slab separately from struct page. This will not be an immediate win, > of course. Indeed, it will likely be a slowdown (overhead of a second > allocation per slab). So there's no urgency to do this until we're > ready to shrink struct page, when we can at least point to that win > as justification. > > Still, we should understand how we're going to get to Page2025 [1] one > step at a time. I had been thinking about coopting compound_head to point > to struct slab. But looking at the places which call folio_test_slab() > [an oxymoron in the New York Interpretation], it becomes apparent that > we need to keep compound_head() and page_folio() working for all pages > for a while. > > As a reminder, compound_head() will _eventually_ return NULL for > slabs & folios. It will only be defined to work for page allocations. > Likewise page_folio() will return NULL for any pages not part of a folio > and page_slab() will return NULL for any pages not part of a slab. > > My best offer right now is to use page->lru.prev. At least one of the > bottom two bits will be set to indicate that it's a memdesc (we're only > going to use thirteen of the memdesc types initially). > Just so I understand it correctly: 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. 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. > 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? -- Cheers David / dhildenb