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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id t19sm9890595ljc.137.2020.08.11.02.37.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 11 Aug 2020 02:37:15 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Tue, 11 Aug 2020 11:37:13 +0200 To: Michal Hocko Cc: Uladzislau Rezki , LKML , RCU , linux-mm@kvack.org, Andrew Morton , Vlastimil Babka , "Paul E . McKenney" , Matthew Wilcox , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag Message-ID: <20200811093713.GB2634@pc636> References: <20200809204354.20137-1-urezki@gmail.com> <20200809204354.20137-2-urezki@gmail.com> <20200810123141.GF4773@dhcp22.suse.cz> <20200810160739.GA29884@pc636> <20200810192525.GG4773@dhcp22.suse.cz> <20200811081917.GG4793@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200811081917.GG4793@dhcp22.suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Tue, Aug 11, 2020 at 10:19:17AM +0200, Michal Hocko wrote: > On Mon 10-08-20 21:25:26, Michal Hocko wrote: > > On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote: > [...] > > > The problem that i see is we can not use the page allocator from atomic > > > contexts, what is our case: > > > > > > > > > local_irq_save(flags) or preempt_disable() or raw_spinlock(); > > > __get_free_page(GFP_ATOMIC); > > > > > > > > > So if we can convert the page allocator to raw_* lock it will be appreciated, > > > at least from our side, IMHO, not from RT one. But as i stated above we need > > > to sort raised questions out if converting is done. > > > > > > What is your view? > > > > To me it would make more sense to support atomic allocations also for > > the RT tree. Having both GFP_NOWAIT and GFP_ATOMIC which do not really > > work for atomic context in RT sounds subtle and wrong. > > I was thinking about this some more. I still think the above would be a > reasonable goal we should try to achieve. If for not other then for > future maintainability (especially after the RT patchset is merged). > I have tried to search for any known problems/attempts to make > zone->lock raw but couldn't find anything. Maybe somebody more involved > in RT world have something to say about that. > I tried yesterday to convert zone->lock. See below files i had to modify: modified: include/linux/mmzone.h modified: mm/compaction.c modified: mm/memory_hotplug.c modified: mm/page_alloc.c modified: mm/page_isolation.c modified: mm/page_reporting.c modified: mm/shuffle.c modified: mm/vmscan.c modified: mm/vmstat.c There is one more lock, that is zone->lru_lock one. Both zone->lock and this one intersect between each other. If the lru_lock can be nested under zone->lock it should be converted as well. But i need to analyze it farther. There are two wrapper functions which are used as common interface to lock/unlock both locks. See compact_lock_irqsave()/compact_unlock_should_abort_lru() in the mm/compaction.c. Any thoughts here? Anyway i tried to convert only zone->lock and use page allocator passing there gfp_mask=0 as argument. So it works. CONFIG_PROVE_RAW_LOCK_NESTING does not complain about any "bad" lock nesting. > Anyway, if the zone->lock is not a good fit for raw_spin_lock then the > only way I can see forward is to detect real (RT) atomic contexts and > bail out early before taking the lock in the allocator for NOWAIT/ATOMIC > requests. > For RT kernel we can detect it for sure. preemtable() works just fine there, i.e. we can identify the context we are currently in. Thanks! -- Vlad Rezki