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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id y13sm991374ljd.19.2020.08.13.02.58.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 13 Aug 2020 02:58:42 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Thu, 13 Aug 2020 11:58:40 +0200 To: Michal Hocko Cc: Thomas Gleixner , paulmck@kernel.org, Uladzislau Rezki , LKML , RCU , linux-mm@kvack.org, Andrew Morton , Vlastimil Babka , 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: <20200813095840.GA25268@pc636> References: <20200811210931.GZ4295@paulmck-ThinkPad-P72> <874kp87mca.fsf@nanos.tec.linutronix.de> <20200813075027.GD9477@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200813075027.GD9477@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 Thu, Aug 13, 2020 at 09:50:27AM +0200, Michal Hocko wrote: > On Wed 12-08-20 02:13:25, Thomas Gleixner wrote: > [...] > > I can understand your rationale and what you are trying to solve. So, if > > we can actually have a distinct GFP variant: > > > > GFP_I_ABSOLUTELY_HAVE_TO_DO_THAT_AND_I_KNOW_IT_CAN_FAIL_EARLY > > Even if we cannot make the zone->lock raw I would prefer to not > introduce a new gfp flag. Well we can do an alias for easier grepping > #define GFP_RT_SAFE 0 > > that would imply nowait semantic and would exclude waking up kswapd as > well. If we can make wake up safe under RT then the alias would reflect > that without any code changes. > > The second, and the more important part, would be to bail out anytime > the page allocator is to take a lock which is not allowed in the current > RT context. Something like > > diff --git a/include/linux/gfp.h b/include/linux/gfp.h > index 67a0774e080b..3ef3ac82d110 100644 > --- a/include/linux/gfp.h > +++ b/include/linux/gfp.h > @@ -237,6 +237,9 @@ struct vm_area_struct; > * that subsystems start with one of these combinations and then set/clear > * %__GFP_FOO flags as necessary. > * > + * %GFP_RT_SAFE users can not sleep and they are running under RT atomic context > + * e.g. under raw_spin_lock. Failure of an allocation is to be expected. > + * > * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower > * watermark is applied to allow access to "atomic reserves" > * > @@ -293,6 +296,7 @@ struct vm_area_struct; > * version does not attempt reclaim/compaction at all and is by default used > * in page fault path, while the non-light is used by khugepaged. > */ > +#define GFP_RT_SAFE 0 > #define GFP_ATOMIC (__GFP_HIGH|__GFP_ATOMIC|__GFP_KSWAPD_RECLAIM) > #define GFP_KERNEL (__GFP_RECLAIM | __GFP_IO | __GFP_FS) > #define GFP_KERNEL_ACCOUNT (GFP_KERNEL | __GFP_ACCOUNT) > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index e028b87ce294..268ae872cc2a 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -2824,6 +2824,13 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order, > { > int i, alloced = 0; > > + /* > + * Hard atomic contexts are not supported by the allocator for > + * anything but pcp requests > + */ > + if (!preemtable()) > + return 0; > + > spin_lock(&zone->lock); > for (i = 0; i < count; ++i) { > struct page *page = __rmqueue(zone, order, migratetype, > @@ -3371,6 +3378,13 @@ struct page *rmqueue(struct zone *preferred_zone, > goto out; > } > > + /* > + * Hard atomic contexts are not supported by the allocator for high > + * order requests > + */ > + if (WARN_ON_ONCE(!preemtable())) > + reurn NULL; > + > /* > * We most definitely don't want callers attempting to > * allocate greater than order-1 page units with __GFP_NOFAIL. > @@ -4523,6 +4537,12 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > (__GFP_ATOMIC|__GFP_DIRECT_RECLAIM))) > gfp_mask &= ~__GFP_ATOMIC; > > + /* Hard atomic contexts support is very limited to the fast path */ > + if (!preemtable()) { > + WARN_ON_ONCE(gfp_mask != GFP_RT_SAFE); > + return NULL; > + } > + > retry_cpuset: > compaction_retries = 0; > no_progress_loops = 0; > > What do you think? > > > which is easy to grep for then having the page allocator go down to the > > point where zone lock gets involved is not the end of the world for > > RT in theory - unless that damned reality tells otherwise. :) > > > > The page allocator allocations should also have a limit on the number of > > pages and eventually also page order (need to stare at the code or let > > Michal educate me that the order does not matter). > > In practice anything but order 0 is out of question because we need > zone->lock for that currently. Maybe we can introduce pcp lists for > higher orders in the future - I have a vague recollection Mel was > playing with that some time ago. > > > To make it consistent the same GFP_ variant should allow the slab > > allocator go to the point where the slab cache is exhausted. > > > > Having a distinct and clearly defined GFP_ variant is really key to > > chase down offenders and to make reviewers double check upfront why this > > is absolutely required. > > Having a high level and recognizable gfp mask is OK but I would really > like not to introduce a dedicated flag. The page allocator should be > able to recognize the context which cannot be handled. > Sorry for jumping in. We can rely on preemptable() for sure, if CONFIG_PREEMPT_RT is enabled, something like below: if (IS_ENABLED_RT && preemptebale()) Also i have a question about pcp-lists. Can we introduce and use all allowed MIGRATE_PCPTYPES? If called with GFP_RT_SAFE? If not please elaborate. According to my tests it really helps when it comes to: succeed(return the page) or NULL. Because on of the list of below types: MIGRATE_UNMOVABLE, MIGRATE_MOVABLE, MIGRATE_RECLAIMABLE, can have a page making allocation succeed. Thanks! -- Vlad Rezki