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b=LyZ6H2/H0XwhRgDFLwozCZOLN2EiPvTtwDpX/dq/KctJfZrNZU0Yb1NL5rBKULsrZ jbUa1UFigmYYxEslnZPJFSFQ0Q0/9PYrAgAXc+BgPZlQSTLpfqhj/L09CSffY8WVX1 xowoiIuBynVFBoupy5E/b30TuKbfNhMRxtSVhcICglhWbHL+46LEDl5T8W4moytKgf oNcgcEHEbUFBhg/WBMRKpzv+KANUEjI+V5nKtsVgfq4Ks5ZVfYbGwz87B/+lgQBcpU 4LnI7IEFZxJbkOGQe8O5rsSp3qYCmn9uvqs+SUbNosbWfqTsnaBeJORpokAocL2Dbl Nmu/fqxw6905A== Message-ID: <627bb280-be44-4648-8771-5a479cda988f@kernel.org> Date: Wed, 13 May 2026 15:02:43 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] mm: introduce a new page type for page pool in page type To: Byungchul Park Cc: Dragos Tatulea , linux-mm@kvack.org, akpm@linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kernel_team@skhynix.com, harry.yoo@oracle.com, ast@kernel.org, daniel@iogearbox.net, davem@davemloft.net, kuba@kernel.org, hawk@kernel.org, john.fastabend@gmail.com, sdf@fomichev.me, saeedm@nvidia.com, leon@kernel.org, tariqt@nvidia.com, mbloch@nvidia.com, andrew+netdev@lunn.ch, edumazet@google.com, pabeni@redhat.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, horms@kernel.org, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, ilias.apalodimas@linaro.org, willy@infradead.org, brauner@kernel.org, kas@kernel.org, yuzhao@google.com, usamaarif642@gmail.com, baolin.wang@linux.alibaba.com, almasrymina@google.com, toke@redhat.com, asml.silence@gmail.com, bpf@vger.kernel.org, linux-rdma@vger.kernel.org, sfr@canb.auug.org.au, dw@davidwei.uk, ap420073@gmail.com References: <20260224051347.19621-1-byungchul@sk.com> <982b9bc1-0a0a-4fc5-8e3a-3672db2b29a1@nvidia.com> <20260513121805.GA22430@system.software.com> <8348d867-f8a1-432c-be2d-699ad96b2e93@kernel.org> <20260513123920.GA51788@system.software.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: <20260513123920.GA51788@system.software.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: DE871180004 X-Stat-Signature: kndp7zfwpgwf43ap1zkiumpw7gw6z7sf X-Rspam-User: X-HE-Tag: 1778677380-599949 X-HE-Meta: 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 jMHsFQxU UuZaKC3dRc036JeLVYYqIHyI9bjJ+JJj6TrXmwKVW19jZmCfLUd/hzr98XOooFFGx0LRbbsmWgRxydSbg7l5Z+XnNMrH8y14jhDb9M8G3jv9OpaUKg6j3FM9ExAAhq2LZ2lyGS44ns7WMNqha/XfX6nAqbRkRW0k8jj7OGVmZ4RAcPKrWjApxvyWaHd0YIg6GVmmC8w51qd2Gf6j8rs60jjJFSZP7een+Me6Pd9luTpVQ8yNLTrjgF9AWwmntykOOZWdTtPBQ8t340uC9hKlv2RDKag4SBAjj1RrTgaTwlj5+b1nZ0R82nh/i8UUsMqmTZRPYhqoTU45Xm+OJonmJvy5HlEMRex6vR8ppagTVtqixwiKgJsIxNAmWRCw26zLN2BlMmPMhpBa2xaloPbdIuSk6Tw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/13/26 14:39, Byungchul Park wrote: > On Wed, May 13, 2026 at 02:29:46PM +0200, David Hildenbrand (Arm) wrote: > On 5/13/26 14:18, Byungchul Park wrote: >>> >>> Hi, >>> >>> The problem comes from the fact that page_type and _mapcount are >>> union'ed but there is a case where these two information should be kept >>> at the same time. >>> >>> Why don't we allow these two information can be kept in the 4 bytes at >>> the same time until Zi Yan's work on _mapcount and page_type will be >>> done, instead of taking a step back? >>> >>> It can be more optimized but I suggest the approach I just mentioned: >>> --- >>> diff --git a/fs/proc/internal.h b/fs/proc/internal.h >>> index 64dc44832808..e5ec204866dc 100644 >>> --- a/fs/proc/internal.h >>> +++ b/fs/proc/internal.h >>> @@ -185,8 +185,7 @@ static inline int folio_precise_page_mapcount(struct folio *folio, >>> { >>> int mapcount = atomic_read(&page->_mapcount) + 1; >>> >>> - if (page_mapcount_is_type(mapcount)) >>> - mapcount = 0; >>> + mapcount = page_mapcount_clear_type(mapcount); >>> if (folio_test_large(folio)) >>> mapcount += folio_entire_mapcount(folio); >>> >>> diff --git a/include/linux/mm.h b/include/linux/mm.h >>> index 8260e28205e9..f45064796313 100644 >>> --- a/include/linux/mm.h >>> +++ b/include/linux/mm.h >>> @@ -1865,8 +1865,7 @@ static inline int folio_mapcount(const struct folio *folio) >>> >>> if (likely(!folio_test_large(folio))) { >>> mapcount = atomic_read(&folio->_mapcount) + 1; >>> - if (page_mapcount_is_type(mapcount)) >>> - mapcount = 0; >>> + mapcount = page_mapcount_clear_type(mapcount); >>> return mapcount; >>> } >>> return folio_large_mapcount(folio); >>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h >>> index 0e03d816e8b9..f3b0d1fa262d 100644 >>> --- a/include/linux/page-flags.h >>> +++ b/include/linux/page-flags.h >>> @@ -934,9 +934,9 @@ static inline bool page_type_has_type(int page_type) >>> } >>> >>> /* This takes a mapcount which is one more than page->_mapcount */ >>> -static inline bool page_mapcount_is_type(unsigned int mapcount) >>> +static inline unsigned int page_mapcount_clear_type(unsigned int mapcount) >>> { >>> - return page_type_has_type(mapcount - 1); >>> + return (unsigned int)(((int)(mapcount << 8)) >> 8); >>> } >>> >>> static inline bool page_has_type(const struct page *page) >>> @@ -953,16 +953,20 @@ static __always_inline void __folio_set_##fname(struct folio *folio) \ >>> { \ >>> if (folio_test_##fname(folio)) \ >>> return; \ >>> - VM_BUG_ON_FOLIO(data_race(folio->page.page_type) != UINT_MAX, \ >>> + VM_BUG_ON_FOLIO(page_type_has_type(data_race(folio->page.page_type)), \ >>> folio); \ >>> - folio->page.page_type = (unsigned int)PGTY_##lname << 24; \ >>> + folio->page.page_type &= ~(PGTY_mapcount_underflow << 24); \ >>> + folio->page.page_type |= (unsigned int)PGTY_##lname << 24; \ >>> } \ >>> static __always_inline void __folio_clear_##fname(struct folio *folio) \ >>> { \ >>> - if (folio->page.page_type == UINT_MAX) \ >>> + int mapcount; \ >>> + \ >>> + if (!page_type_has_type(folio->page.page_type)) \ >>> return; \ >>> VM_BUG_ON_FOLIO(!folio_test_##fname(folio), folio); \ >>> - folio->page.page_type = UINT_MAX; \ >>> + mapcount = atomic_read(&folio->page._mapcount); \ >>> + folio->page.page_type = page_mapcount_clear_type(mapcount); \ >>> } >>> >>> #define PAGE_TYPE_OPS(uname, lname, fname) \ >>> @@ -975,15 +979,20 @@ static __always_inline void __SetPage##uname(struct page *page) \ >>> { \ >>> if (Page##uname(page)) \ >>> return; \ >>> - VM_BUG_ON_PAGE(data_race(page->page_type) != UINT_MAX, page); \ >>> - page->page_type = (unsigned int)PGTY_##lname << 24; \ >>> + VM_BUG_ON_PAGE(page_type_has_type(data_race(page->page_type)), \ >>> + page); \ >>> + page->page_type &= ~(PGTY_mapcount_underflow << 24); \ >>> + page->page_type |= (unsigned int)PGTY_##lname << 24; \ >>> } \ >>> static __always_inline void __ClearPage##uname(struct page *page) \ >>> { \ >>> - if (page->page_type == UINT_MAX) \ >>> + int mapcount; \ >>> + \ >>> + if (!page_type_has_type(page->page_type)) \ >>> return; \ >>> VM_BUG_ON_PAGE(!Page##uname(page), page); \ >>> - page->page_type = UINT_MAX; \ >>> + mapcount = atomic_read(&page->_mapcount); \ >>> + page->page_type = page_mapcount_clear_type(mapcount); \ >>> } >>> >>> /* >>> diff --git a/mm/debug.c b/mm/debug.c >>> index 77fa8fe1d641..9a932ded09d4 100644 >>> --- a/mm/debug.c >>> +++ b/mm/debug.c >>> @@ -74,8 +74,7 @@ static void __dump_folio(const struct folio *folio, const struct page *page, >>> int mapcount = atomic_read(&page->_mapcount) + 1; >>> char *type = ""; >>> >>> - if (page_mapcount_is_type(mapcount)) >>> - mapcount = 0; >>> + mapcount = page_mapcount_clear_type(mapcount); >>> >>> pr_warn("page: refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n", >>> folio_ref_count(folio), mapcount, mapping, >>> --- >>> >>> Thoughts? >> >> God no. > > This is not final patch, but for sharing the rough idea *with code* - > maybe there are more points in code that should be adjusted by the > change. I just typed the draft patch quick just for sharing idea. > > If we should allow pp type pages to be used in mapping as well, then > we should allow a page to keep both its type and mapcount at the same > time. Am I missing something? We don't want code to accidentally overflow mapcounts into these bits and have them wrongly be detected as page types. This is just very fragile. -- Cheers, David