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Thu, 11 Sep 2025 07:10:49 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id uC4gA/l1wmghVwAAD6G6ig (envelope-from ); Thu, 11 Sep 2025 07:10:49 +0000 Message-ID: <08e7cac0-704e-48c4-ba13-c857dc1d11f0@suse.cz> Date: Thu, 11 Sep 2025 09:10:48 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 06/22] slab: Reimplement page_slab() To: "Christoph Lameter (Ampere)" , "Matthew Wilcox (Oracle)" Cc: Andrew Morton , David Rientjes , Roman Gushchin , Harry Yoo , linux-mm@kvack.org References: <20250910115507.1991829-1-willy@infradead.org> <20250910115507.1991829-7-willy@infradead.org> <8eefb8ca-e310-48d4-0c3a-fa80665cab87@gentwo.org> Content-Language: en-US From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: <8eefb8ca-e310-48d4-0c3a-fa80665cab87@gentwo.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspamd-Queue-Id: ED662180006 X-Rspamd-Server: rspam05 X-Stat-Signature: t4h1edawu68ms1mbf7sbkyiub4n7xgau X-Rspam-User: X-HE-Tag: 1757574650-827470 X-HE-Meta: U2FsdGVkX19ZgSPOFnGJt1e+DwAr5iIal/Kkr66LeOiZlxnBogFlf68hrwW7+naUPMggd5nxChz0SkLOWNCBgnZRGJfl+aCqSQbSLNkM2dYXJLhLGXr2xSmWSbmdNILgyrK0bhv7tTh3PtxLmuB0FMwDzDtKHG9q59dR5XN5pbyfV7ku6/UW1/qrvNDaAVaaq9br2+mG62VhA4AGk4dHm7HmGCKrruiYqT5/EBipIsHGzPgR3EAQxyB84OifXj2YyiDsl2ibe8CGfLWk8GYA70NSjlinuERi+HZIGA7PpVvJbgYmToo9dhTwlaEaj0jZMiza3ftgRf4c4jY4Jazz7ejAIYIxs6vwp183Jc5PWUb/fR8+YSxOlbbhnqMmU4GVC9kK6S0cOVubGKVO71IJik1MJs5VbYB50R4e+wg3i3AlJ5Xyyh+xyBXYZRvNSzWQkIUsvY50FeJD50OHALcL9iiHZG9OMH6q04p88iJyEvdDfsxwaCy4vB5qpsfe5moVTw4tDI2xCXjy170V7F18xXFNku0ugFzIkuf1L1FlmUQYerisfFr5A9fjeo8hKquIm4iLiylUzs4U7Pyl9lbpUTznG0XlnARj+RVn1qROSVRGHAJRcMhaIyDH9XNQWtG7ehd12NVcOWr0mzElZ8RHtCB9XKvPQYHs5v5KscmrUpDx0yuhIF/r6cNq5Jb4Bs9u+iDbWc2ED2SRRe0DUz7bxzvakJjGXx74q2tPLSBThrpv+z46aLtZ2oKJ6KmTh3GYAtCXlPFCwCvzLYG21E3MUgU6HRIWHSw0KvJ53d1vKNjP07n4Q8dfD/rWypjEs7qpgTYtKljuSfBfIIIQgGKq3RbF2PRN3XRl+83G+rfHpud39JR8xv9UoCl5Dnxwno2uf0VsPx5BWNjgrCYSjQ3fq9pIfCUIlj7yuVeG0ReeGEJ/XLD5swNRHe6O+w1TvCZH0djUtB8d9z9wKJNBVXN rAHrMnuz nd36aovtp6NFfSWGD/l683vD584uWbWbCQx0JtPmI+MT7z/7r5KIfZA4Rf28rXqrmcdu7gSBesYiEaRs72aJa03ffIBy6QouUB8k6nnfyEjCNEFrb7ypgEE1AcYjH2iomWDl8QVNM74Tw8xBScor0MhclA5A9fAIgEu98bmqfairMUkLt3e3BIUtnqRygiGfrq8aFpK99QIoIk97VXQ88+EPHxx5x5MyL53qQ591ZVpKadhmzOlwqTzqeL3H+dACRcd3qebAgEsF74SoCHl2o+Bbo0PFa2xzkwrHRU0SZRHrBCXxHH+Zr1lXriVpJgXTAQSlImkBFhE+vLGI= 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 9/11/25 00:46, Christoph Lameter (Ampere) wrote: > On Wed, 10 Sep 2025, Matthew Wilcox (Oracle) wrote: > >> In order to separate slabs from folios, we need to convert from any page >> in a slab to the slab directly without going through a page to folio >> conversion first. page_slab() is a little different from other memdesc >> converters we have in that it will return NULL if the page is not part >> of a slab. This will be the normal style for memdesc converters in >> the future. > > We are looking forward to another level of indirection in order > to get to struct slab? > >> -#define page_slab(p) (_Generic((p), \ >> - const struct page *: (const struct slab *)(p), \ >> - struct page *: (struct slab *)(p))) >> +static inline struct slab *page_slab(const struct page *page) >> +{ >> + unsigned long head; >> + >> + head = READ_ONCE(page->compound_head); >> + if (head & 1) >> + page = (struct page *)(head - 1); >> + if (data_race(page->page_type >> 24) != PGTY_slab) >> + page = NULL; >> + >> + return (struct slab *)page; >> +} > > compound_head() is open coded in there. But since this is replacing going through page_folio() (in the following patches) that does compound_head() itself, we're not really adding a new indirection, no? Except to kfence here which knows the page pointer is head already and now we're adding an unnecessary compound_head(). But it affects only kfence_init_pool() and not any fastpaths, AFAICS. I guess later with memdescs we can have a scheme where all head and tail memdescs could contain the struct slab pointer so we don't have go through the head one first? Which would replace one indirection with another and not add a new one either.