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From: Shakeel Butt To: Vlastimil Babka Cc: Suren Baghdasaryan , Hao Ge , Alexei Starovoitov , Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Harry Yoo , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hao Ge , David Hildenbrand Subject: Re: [PATCH v3] slab: Add check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg_kmem Message-ID: References: <20251014152751.499376-1-hao.ge@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: A4C001C0003 X-Rspamd-Server: rspam03 X-Stat-Signature: bjyifwor8gf6wqaxhe8sh579iu5uis4t X-HE-Tag: 1760481660-859205 X-HE-Meta: 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 0lUIXh3e T6cpyzW9NQ4SAKG5V7PiInN4eL6mgmiCe91XEHVjqne2NM4RYhaFhX97kxNZugaZs1l8TKFbUV7X1VbK06j8IpscUpv/kMYcpwryauMvLOUGEnO/DiMRHiukySS9mTcZxiOg+ziQHjVKA6ln+Lxs2ZfN+nn9/NO8uFZp8Z3L29kkRPSpvTufyvW+IOPVzlVvw/Io7KJLQfNDj+EjpUj53fA3Bqf3AAir0uOfQ4DKcT+gtF8Xv0C0wJyWGywFTv1lsi5WoMuolzzD5ZAd3VwTIhpfXPSm8QeWibKjMp4YyNjPj5Pr7568hV4HFmaiINtwx1eoOfryBOCnx6WAXIQXr48B+OHeBD1Mi/wvO7pPTWKwAXAyhYieJMbN1eaOelRsjst5onz5sXw0YK48= 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 Tue, Oct 14, 2025 at 10:58:25PM +0200, Vlastimil Babka wrote: > On 10/14/25 22:14, Shakeel Butt 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. > >> > > >> > 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. > > > > With Suren's suggestion, you can add: > > > > Acked-by: Shakeel Butt > > Thanks, I added Suren's suggestion and pushed to slab/for-next-fixes: > > https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/commit/?h=slab/for-next-fixes&id=711c435c89e59ee32bf8bb1c0d875a07931da5a8 > > Resisted the impulse to change the single VM_BUG_ON_FOLIO to > VM_WARN_ON_ONCE_FOLIO because we're still going to do that systematically, > right? Oh is there some coordinated effort happening for this conversion?