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Wed, 20 Mar 2024 13:40:05 +0900 (JST) (envelope-from penguin-kernel@I-love.SAKURA.ne.jp) Message-ID: Date: Wed, 20 Mar 2024 13:40:05 +0900 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm,page_owner: Fix refcount imbalance Content-Language: en-US To: Andrew Morton , Oscar Salvador Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Michal Hocko , Vlastimil Babka , Marco Elver , Andrey Konovalov , Alexander Potapenko , syzbot+41bbfdb8d41003d12c0f@syzkaller.appspotmail.com References: <20240319183212.17156-1-osalvador@suse.de> <20240319183212.17156-2-osalvador@suse.de> <20240319162430.3e76504d06f12edc107029a4@linux-foundation.org> From: Tetsuo Handa In-Reply-To: <20240319162430.3e76504d06f12edc107029a4@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 8877340011 X-Rspam-User: X-Stat-Signature: psrjgo9xf7f6jnunzk4ucmb1tnbxy486 X-Rspamd-Server: rspam01 X-HE-Tag: 1710909620-453342 X-HE-Meta: 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 A3epMH+p LsUir/aZva92deTtNkx6r86xOmkjgHSVpXhfW5UkIObHeppU= 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 2024/03/20 8:24, Andrew Morton wrote: > On Tue, 19 Mar 2024 19:32:11 +0100 Oscar Salvador wrote: >> -static void dec_stack_record_count(depot_stack_handle_t handle) >> +static void dec_stack_record_count(depot_stack_handle_t handle, >> + int nr_base_pages) >> { >> struct stack_record *stack_record = __stack_depot_get_stack_record(handle); >> >> if (stack_record) >> - refcount_dec(&stack_record->count); >> + refcount_sub_and_test(nr_base_pages, &stack_record->count); >> } > > mm/page_owner.c: In function 'dec_stack_record_count': > mm/page_owner.c:226:17: error: ignoring return value of 'refcount_sub_and_test' declared with attribute 'warn_unused_result' [-Werror=unused-result] > 226 | refcount_sub_and_test(nr_base_pages, &stack_record->count); > | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > cc1: all warnings being treated as errors > Hmm, I guess that this is not an expected user of refcount API. If it is correct behavior that refcount becomes 0 here, you need to explain like - refcount_sub_and_test(nr_base_pages, &stack_record->count); + if (refcount_sub_and_test(nr_base_pages, &stack_record->count)) { + // Explain why nothing to do here, and explain where/how + // refcount again becomes positive value using refcount_set(). + } or replace refcount_t with atomic_t where it is legal to make refcount positive without using atomic_set().