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From: Shakeel Butt To: Suren Baghdasaryan Cc: Hao Ge , Vlastimil Babka , 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 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-Rspamd-Server: rspam01 X-Stat-Signature: qoxtpdgrgby1q6wrd5r54gi3qzpas181 X-Rspam-User: X-Rspamd-Queue-Id: EA3DE20011 X-HE-Tag: 1760472915-199189 X-HE-Meta: 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 RZ14jR+q ncz2MKXQYft0MLrH3/QcsB+7XTvfrwXlEDZZ4/pN/3RSXnf+0ICIzsMCyrKmhVfmAWAsGxRxbjPiaHqYk25xaRcE1bOfFHmDE3JMy20DsBNYyRlkbwtN1nYs+VWzt2HYTZUbL0ZYRiH4vHS4oRTqRwV58doV40Adan/xt6FswS0cca7ajKMXlEQooMYOOJ8DdzwlRhAjKDcULxALFMoOJlXlYmasEyC1i0ueno4C2dIs7m05UZnxjGpma2ZuCUQKudOn2KAJM0b5KQk8TAdgkpxJBHoybdfGc8n1fh83iPQiqi3J1O5KyOSTujiwnphPWxRGlePdcjysv6ot67c+vjPz4X9xNfdxxRWhCbZi6Zmio0bSmUqM7+yIB8Q== 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 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