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Wed, 15 Oct 2025 10:37:14 +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 r0ogHFp572jVHgAAD6G6ig (envelope-from ); Wed, 15 Oct 2025 10:37:14 +0000 Message-ID: <6194b6ab-6b43-468d-ba78-a95594c48798@suse.cz> Date: Wed, 15 Oct 2025 12:37:14 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] slab: Add check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg_kmem Content-Language: en-US To: Harry Yoo Cc: Suren Baghdasaryan , Hao Ge , Alexei Starovoitov , Andrew Morton , Johannes Weiner , Shakeel Butt , Michal Hocko , Roman Gushchin , Muchun Song , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hao Ge References: <20251014152751.499376-1-hao.ge@linux.dev> 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: 42A652000A X-Rspamd-Server: rspam02 X-Stat-Signature: a51f81szb8xwsk78g6bcuthp3doddu1z X-HE-Tag: 1760524636-693068 X-HE-Meta: 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 t7UP5cSD akTSH67vY+QDp/3K7pbFBnRV5IKD3uZEG62me4aqLKmeaUaXTLFrcCqIDKh9pJSQ6axLXwvnaAGB6yM/A41IX8eIrozkDfKNE6bOO6SCLgwUvrAfaI/wSK1xU46X1GKbmNA9zIhTbS2L5ScZT2Y2w+l6QCeodFaiNsXL95wTNbzYbJIcp3tLN+aei5EvnOofYgZC/1Ji5f22VlTrifz9FI68EYniz0LbsCgbLmajaSm8lhgbqDSmZIjCTv3T3GLhJHiIZBH1T//vMY/OSMJig8CuvY3fXIjCo6bNMjOb0PmslVQfK6s4hNRS/f1qC6XflxtkotX9Vpm6BtYVNlt0nuBBFplhT40wUifpgGzxq6yHP9e3ccO49qIVKQC7/gjyEUno9j2Z5WQuDRO9UASfdH/t0tcjThuy2c5yC3VTNZExmcPNp7CQoPuXORQj7EVdEVdyJ 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 10/15/25 12:27, Harry Yoo wrote: > On Wed, Oct 15, 2025 at 11:54:18AM +0200, Vlastimil Babka wrote: >> On 10/15/25 11:25, Harry Yoo wrote: >> > On Tue, Oct 14, 2025 at 09:12:43AM -0700, Suren Baghdasaryan wrote: >> >> On Tue, Oct 14, 2025 at 8:28 AM Hao Ge wrote: >> >> > >> >> > From: Hao Ge >> >> > >> >> > Since OBJEXTS_ALLOC_FAIL and MEMCG_DATA_OBJEXTS currently share >> >> > the same bit position, we cannot determine whether memcg_data still >> >> > points to the slabobj_ext vector simply by checking >> >> > folio->memcg_data & MEMCG_DATA_OBJEXTS. >> >> > >> >> > If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL, >> >> > and during the release of the associated folio, the BUG check is triggered >> >> > because it was mistakenly assumed that a valid folio->memcg_data >> >> > was not cleared before freeing the folio. >> > >> > nit: yesterday I was confused that this is sanity checks in buddy complaining >> > folio->memcg_data not being cleared, but it's actually folio_memcg_kmem() >> > complaining that MEMCG_OBJEXTS_DATA flag is set on non-slab folios (in >> > free_pages_prepare(), if PageMemcgKmem(page) -> __memcg_kmem_uncharge_page())) >> > So the paragraph above should be updated? >> > >> > And as a side question, we clear slab->obj_exts when freeing obj_ext array, >> > but don't clear OBJEXTS_ALLOC_FAIL when freeing a slab? That's not good. >> >> Hm great point. We should rather make sure it's cleared always, instead of >> adjusting the debugging check, which shouldn't be then necessary, right? > > Yeah folio_memcg_kmem() isn't supposed to be called on slabs anyway > (it's not a slab at the time we free it to buddy), so we don't have to > adjust the debug check. Great. Hao Ge, can you please send v4 that instead of adjusting the VM_BUG_ON modifies free_slab_obj_exts() to always clear slab->obj_exts? Thanks! >> >> > So let's check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg_kmem. >> >> > >> >> > Fixes: 7612833192d5 ("slab: Reuse first bit for OBJEXTS_ALLOC_FAIL") >> >> > Suggested-by: Harry Yoo >> >> > Signed-off-by: Hao Ge >> >> >> >> Reviewed-by: Suren Baghdasaryan >> >> >> >> nit: I think it would be helpful if the changelog explained why we >> >> need the additional check. We can have the same bit set in two >> >> different situations: >> >> 1. object extension vector allocation failure; >> >> 2. memcg_data pointing to a valid mem_cgroup. >> >> To distinguish between them, we need to check not only the bit itself >> >> but also the rest of this field. If the rest is NULL, we have case 1, >> >> otherwise case 2. >> > >> > Agreed. >> > >> > In general LGTM, >> > Reviewed-by: Harry Yoo >> > >> > By the way, maybe it'd be nice to introduce a new helper function that >> > properly checks MEMCG_DATA_OBJEXTS flag. >> >> I thought so too at first... >> >> >> ~/slab (slab/for-next-fixes)> git grep -n MEMCG_DATA_OBJEXTS >> >> include/linux/memcontrol.h:337: MEMCG_DATA_OBJEXTS = (1UL << 0), >> >> include/linux/memcontrol.h:344:#define __OBJEXTS_ALLOC_FAIL MEMCG_DATA_OBJEXTS >> >> include/linux/memcontrol.h:358: * MEMCG_DATA_OBJEXTS. >> > >> >> include/linux/memcontrol.h:400: VM_BUG_ON_FOLIO(memcg_data & MEMCG_DATA_OBJEXTS, folio); >> >> include/linux/memcontrol.h:421: VM_BUG_ON_FOLIO(memcg_data & MEMCG_DATA_OBJEXTS, folio); >> > >> > these two, >> > >> >> include/linux/memcontrol.h:492: if (memcg_data & MEMCG_DATA_OBJEXTS) >> > >> > this, >> > >> >> include/linux/memcontrol.h:538: (folio->memcg_data & MEMCG_DATA_OBJEXTS), >> >> include/linux/memcontrol.h:1491: * if MEMCG_DATA_OBJEXTS is set. >> >> mm/kfence/core.c:624: MEMCG_DATA_OBJEXTS; >> > >> >> mm/page_owner.c:513: if (memcg_data & MEMCG_DATA_OBJEXTS) >> > >> > this, >> > >> >> mm/slab.h:541: * MEMCG_DATA_OBJEXTS bit set or be equal to OBJEXTS_ALLOC_FAIL. >> >> mm/slab.h:543: VM_BUG_ON_PAGE(obj_exts && !(obj_exts & MEMCG_DATA_OBJEXTS) && >> >> mm/slub.c:2137: new_exts |= MEMCG_DATA_OBJEXTS; >> >> tools/mm/show_page_info.py:55: MEMCG_DATA_OBJEXTS = prog.constant("MEMCG_DATA_OBJEXTS").value_() >> > >> >> tools/mm/show_page_info.py:59: if memcg_data & MEMCG_DATA_OBJEXTS: >> > >> > and this do not look good. >> > >> > I mean technically they are fine since OBJEXTS_ALLOC_FAIL is set on >> > slabs only, but that's just a coincidence. >> >> And checked the the other debugging checks too. But then thought it's better >> that if these are not expected to see slabs, then they should not be >> adjusted. I don't see it as a coincidence but as intention to keep it slab >> specific. It will be also more future proof for the upcoming separation of >> struct slab from struct page. > > Then we're intentionally using (folio->memcg_data & MEMCG_DATA_OBJEXTS) check > as a way to determine whether the folio is a slab (either slabobj_ext array > allocation succeeded or failed). > > That makes sense to me! > >> >> > --- >> >> > v3: Simplify the solution, per Harry's suggestion in the v1 comments >> >> > Add Suggested-by: Harry Yoo >> >> > --- >> >> > include/linux/memcontrol.h | 4 +++- >> >> > 1 file changed, 3 insertions(+), 1 deletion(-) >> >> > >> >> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h >> >> > index 873e510d6f8d..7ed15f858dc4 100644 >> >> > --- a/include/linux/memcontrol.h >> >> > +++ b/include/linux/memcontrol.h >> >> > @@ -534,7 +534,9 @@ static inline struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *ob >> >> > static inline bool folio_memcg_kmem(struct folio *folio) >> >> > { >> >> > VM_BUG_ON_PGFLAGS(PageTail(&folio->page), &folio->page); >> >> > - VM_BUG_ON_FOLIO(folio->memcg_data & MEMCG_DATA_OBJEXTS, folio); >> >> > + VM_BUG_ON_FOLIO((folio->memcg_data != OBJEXTS_ALLOC_FAIL) && >> >> > + (folio->memcg_data & MEMCG_DATA_OBJEXTS), >> >> > + folio); >> >> > return folio->memcg_data & MEMCG_DATA_KMEM; >> >> > } >> >> > >> >> > -- >> >> > 2.25.1 >> > >> >