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b=S5OCbYapRm0km9uPuedMjFuUoxnXo8KQTa3IlLYLUskH3GYCpqf52og9RaA8UQkPc 1YxdPmzOzNe7mvsoOMzDXvnRrkjtWMZFF+2op7RselwwXEfRU+mMrbpqfivhdCcfMK TS9anlNRbaPJAQf0+vjOKPpRhfSJjBMSC3FFd93WG81HQfuemV4ff1F0QaIRMy3EoP NKG8/JodK9AiefK/PsvbLZFPQc3qXkKdxsgqfwtku4zEBAokFTk9PKhU/Oa52Cu0Gq eFJNzvvvT9IR7FJhJwdNOxOQ5w4hdHZ/U35ix9bEH1k7s2Cf0YaVLL2YB9XcIoc9AR 6kCyHK2T6WIrg== Message-ID: Date: Tue, 25 Aug 2026 10:31:34 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/4] Extend xas_split* to support splitting arbitrarily large entries To: Matthew Wilcox , Artem Lytkin Cc: Ackerley Tng , Zi Yan , Kiryl Shutsemau , Hugh Dickins , Baolin Wang , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org References: <20251117224701.1279139-1-ackerleytng@google.com> <20260824194755.216000-1-iprintercanon@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+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 Content-Transfer-Encoding: 7bit X-Stat-Signature: itffw48e5jrfc7mu9ngramxzd6n9bba6 X-Rspamd-Queue-Id: D890AA000B X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1787646699-909922 X-HE-Meta: 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 tEhlL/pY 1jBg+zigNzLDVwfvopovxqxEwObuHjNGMTXztKeMHwkqJQhx7L+tOvaOlL+iBtvinxSTALQ6hHZuPfHEgSMO5+lBqzdNb4ZMleB+dyfV7L4XVDjqH7WMEBJp/rHlo3VvXAhuKTLXOdgZ4O+Ve7SykKi13mBQ80nU7r9lOMoqhaXQACKHEVVvrLZXgh3+SithlFFjkbDMSfmjR79lVJFFC6atSjkMGciOLDwzVersOUaIDswNv2iWRlTeCq5812EPOPPlXMUujhEpAFLjGx1lR+IfiJUF55x4B4I/7u+0u8nYXCiyLfxqbYVs0ig== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/24/26 22:31, Matthew Wilcox wrote: > On Mon, Aug 24, 2026 at 10:47:55PM +0300, Artem Lytkin wrote: >> I'm asking because there is a second user for it that has nothing to do >> with guest_memfd. On arm64 with 64K pages HPAGE_PMD_ORDER is 13 and >> MAX_XAS_ORDER is 11, so MAX_PAGECACHE_ORDER clamps to 11 and shmem never >> offers a PMD order. MADV_COLLAPSE on tmpfs returns -EINVAL there, and >> huge=always hands out order-11 folios at best, so ShmemPmdMapped never >> leaves zero. Every other page cache file system tops out at order 11 as >> well, which is 128M and not PMD-mappable. pagemap.h admits it outright: >> "xas_split_alloc() does not support arbitrary orders. This implies no >> 512MB THP on ARM64 with 64KB base page size." Kiryl's khugepaged >> selftest series skips the shmem and file cases altogether for exactly >> this reason, and on 6 August he confirmed in that thread that uniform >> splits still go through xas_split_alloc(), so xas_try_split() on its >> own does not lift the cap. > > But why would you want it? With 64KiB pages on arm64, you need 512MiB > for a PMD-sized page, and that's just too big for current hardware. > What's your bandwidth? 40GB per DIMM? With 8 DIMMs, that's an > aggregate of 320GB bandwidth, so that restricts you to 640 page faults > per second. That's too few for almost all workloads. > > The page allocator also isn't great at doing order-13 allocations. I heard some use cases using 512M THPs on arm64 to back VMs, but it's mostly anon pages, and not really ordinary pagecache. > > Since arm64 does support 2MiB TLBs, even with a 512MiB PMD size, I'm > not sure there's a huge performance win waiting for you on the other > side of all this work. > > IOW is this driven by a real need, or just some completionist thing? > >> That case needs two new levels of nodes rather than one: 130 nodes, >> 73 KiB, a long way from the 1G to 4K split Matthew was worried about. >> Raising MAX_XAS_ORDER to 3 * XA_CHUNK_SHIFT - 1 covers it, and >> MAX_PAGECACHE_ORDER stays min()'d with HPAGE_PMD_ORDER, so no page cache >> caller can ask for the order-17 worst case that cap admits. > > 130 nodes is still a lot to ask the slab allocator for! With 64KiB > pages it's not too bad -- you get 112 per page. But still ... IMHO it would be nice if we could just support such splits. I've always been thinking of it like this: you end up consuming the exact same memory if you wouldn't have allocated THPs in the first place. But a buddy-like split is clearly preferable. (or avoiding splitting in the first place :) ) -- Cheers, David