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Mon, 9 Jun 2025 14:22:02 +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 lKs+CQruRmhBbQAAD6G6ig (envelope-from ); Mon, 09 Jun 2025 14:22:02 +0000 Message-ID: Date: Mon, 9 Jun 2025 16:22:01 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/slab: fix folio_test_{anon,ksm}() false positive on slabs Content-Language: en-US To: Harry Yoo , rientjes@google.com, cl@gentwo.org, akpm@linux-foundation.org Cc: roman.gushchin@linux.dev, willy@infradead.org, linux-mm@kvack.org References: <20250609132723.13118-1-harry.yoo@oracle.com> 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: <20250609132723.13118-1-harry.yoo@oracle.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Stat-Signature: xsgaifp5b31irrsgkhquicuzyrrb5pyh X-Rspamd-Queue-Id: 7139FC0007 X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1749478925-549220 X-HE-Meta: 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 uar4RsYh nX81lIfOBvP4YJ3LhwpkVGFqhHRch90Y3I0qUq/ol5+fKmLMBLkGNm96rjMpD0oQxrWh0ztOwekMsHFyuJZ3XTgwzOrQaOA3Bc9m8yKLJXuTmXhzuFZXPL7vYfHXXvKT+AeJD6tMRh+tNw3m0cKgtF4Vw8bOMIYCkWP5TvBOdMwGyqOUOxJEUyubl0aCHwbvlDRyQG6KcRaas1H8CtVD0CcJZ8mruSwtBndK4ZdvlBTadVxmf1FWdMDUFNktZPxafVN4pM+YJyiwV7DVjjIAzYG5gxPjA4jE74I1xRVGMMlYIaTix/Q/uoXVuUcL/+XqUs2Rp1wqw4CxVdBt+H4x3QEHGGFDy44J7hg/c 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 6/9/25 15:27, Harry Yoo wrote: > When running tools/mm/page-types (with flags, page-counts, MB omitted), > it reports that some slabs have KPF_ANON and KPF_KSM set: > > $ sudo ./page-types | grep slab > _______S___________________________________ slab > _______S____a________x_____________________ slab,anonymous,ksm > > This is unexpected, as slab memory should never be treated as anonymous > memory. This is because slab->slabs shares the same offset as > page->mapping and setting the lower two bits of ->slabs field means > folio_test_anon() and folio_test_ksm() will return true on the slab: > > [ field ] [ offset ] [ size ] > page->mapping 24 8 > page->lru 8 16 > > slab->next 16 8 > slab->slabs 24 4 > > Reorder ->slabs and ->next, so that the layout will be: > > slab->slabs 16 4 > slab->next 24 8 > > After reordering, slab->slabs shares its offset with page->lru.prev, > which is not a problem. slab->next now shares the offset with > page->mapping, but that's fine, as a slab is double-word aligned. I think the double-word alignment doesn't even need to be mentioned, these mapping flags are fine with just natural pointer's alignment that means the lowest two bits are 0? > > With the change, the nonsense slab pages disappear: > > $ sudo ./page-types | grep slab > _______S___________________________________ slab > > Fixes: 130d4df57390 ("mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head") > Signed-off-by: Harry Yoo > --- > > No Cc: stable because we don't usually check folio_test_anon() on slabs. Hmm maybe we could just make the code generating /proc/kpageflags stop testing/reporting those "flags" for slab pages? It could be even more future proof code for when struct slab is separated. I mean there's even a comment above PAGE_MAPPING_ANON: * For slab pages, since slab reuses the bits in struct page to store its * internal states, the folio->mapping does not exist as such, nor do * these flags below. So in order to avoid testing non-existent bits, * please make sure that folio_test_slab(folio) actually evaluates to * false before calling the following functions (e.g., folio_test_anon). * See mm/slab.h. Otherwise the change seems safe but maybe we're missing some other corner case :) Willy what do you think? > mm/slab.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/slab.h b/mm/slab.h > index 05a21dc796e0..2e7064f7709e 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -59,8 +59,8 @@ struct slab { > struct list_head slab_list; > #ifdef CONFIG_SLUB_CPU_PARTIAL > struct { > - struct slab *next; > int slabs; /* Nr of slabs left */ > + struct slab *next; > }; > #endif > };