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Mon, 9 Aug 2021 20:08:45 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id uC5ZC02LEWG3fQAAMHmgww (envelope-from ); Mon, 09 Aug 2021 20:08:45 +0000 Subject: Re: [PATCH v4 29/35] mm: slub: Move flush_cpu_slab() invocations __free_slab() invocations out of IRQ context To: Mike Galbraith , Qian Cai , Andrew Morton , Christoph Lameter , David Rientjes , Pekka Enberg , Joonsoo Kim Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sebastian Andrzej Siewior , Thomas Gleixner , Mel Gorman , Jesper Dangaard Brouer , Jann Horn References: <20210805152000.12817-1-vbabka@suse.cz> <20210805152000.12817-30-vbabka@suse.cz> <0b36128c-3e12-77df-85fe-a153a714569b@quicinc.com> <6e3bb54fdf7208f3f86a5d31e1ef2a576d8e7e37.camel@gmx.de> From: Vlastimil Babka Message-ID: <7d9ac614-f18e-de5d-2d47-f92e618d76fc@suse.cz> Date: Mon, 9 Aug 2021 22:08:44 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.12.0 MIME-Version: 1.0 In-Reply-To: <6e3bb54fdf7208f3f86a5d31e1ef2a576d8e7e37.camel@gmx.de> Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: C5001B000191 Authentication-Results: imf19.hostedemail.com; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=pqB99FiB; dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=V4x9XN4P; dmarc=none; spf=pass (imf19.hostedemail.com: domain of vbabka@suse.cz designates 195.135.220.28 as permitted sender) smtp.mailfrom=vbabka@suse.cz X-Stat-Signature: qnexrakk7ajcpqz6xdxzmfquh61yg6k1 X-HE-Tag: 1628539726-51061 Content-Transfer-Encoding: quoted-printable 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: On 8/9/2021 8:44 PM, Mike Galbraith wrote: > On Mon, 2021-08-09 at 09:41 -0400, Qian Cai wrote: >> >> >> On 8/5/2021 11:19 AM, Vlastimil Babka wrote: >>> >>> =C2=A0 >>> +static DEFINE_MUTEX(flush_lock); >>> +static DEFINE_PER_CPU(struct slub_flush_work, slub_flush); >>> + >>> =C2=A0static void flush_all(struct kmem_cache *s) >>> =C2=A0{ >>> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0on_each_cpu_cond(has_cpu_s= lab, flush_cpu_slab, s, 1); >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0struct slub_flush_work *sf= w; >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0unsigned int cpu; >>> + >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0mutex_lock(&flush_lock); >> >> Vlastimil, taking the lock here could trigger a warning during memory >> offline/online due to the locking order: >> >> slab_mutex -> flush_lock >=20 > Bugger. That chain ending with cpu_hotplug_lock makes slub_cpu_dead() > taking slab_mutex a non-starter for cpu hotplug as well. It's > established early by kernel_init_freeable()..kmem_cache_destroy() as > well as by slab_mem_going_offline_callback(). I suck at reading the lockdep splats, so I don't see yet how the "existin= g reverse order" occurs - I do understand the order in the "lsbug". What I also wonder is why didn't this occur also in the older RT trees wi= th this patch. I did change the order of locks in flush_all() to take flush_lock = first and cpus_read_lock() second, as Cyrill Gorcunov suggested. Would the orig= inal order prevent this? Or we would fail anyway because we already took cpus_read_lock() in offline_pages() and now are taking it again - do thes= e nest or not?