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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id l188sm43120lfd.127.2020.10.06.15.25.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 06 Oct 2020 15:25:31 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Wed, 7 Oct 2020 00:25:29 +0200 To: Michal Hocko Cc: Uladzislau Rezki , Mel Gorman , Vlastimil Babka , LKML , RCU , linux-mm@kvack.org, Andrew Morton , "Paul E . McKenney" , Peter Zijlstra , Thomas Gleixner , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func. Message-ID: <20201006222529.GA23612@pc636> References: <20200918194817.48921-3-urezki@gmail.com> <38f42ca1-ffcd-04a6-bf11-618deffa897a@suse.cz> <20200929220742.GB8768@pc636> <795d6aea-1846-6e08-ac1b-dbff82dd7133@suse.cz> <20201001192626.GA29606@pc636> <20201002071123.GB20872@dhcp22.suse.cz> <20201002085014.GC3227@techsingularity.net> <20201002090507.GB4555@dhcp22.suse.cz> <20201005150801.GC17959@pc636> <20201005154100.GF4555@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201005154100.GF4555@dhcp22.suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 05, 2020 at 05:41:00PM +0200, Michal Hocko wrote: > On Mon 05-10-20 17:08:01, Uladzislau Rezki wrote: > > On Fri, Oct 02, 2020 at 11:05:07AM +0200, Michal Hocko wrote: > > > On Fri 02-10-20 09:50:14, Mel Gorman wrote: > > > > On Fri, Oct 02, 2020 at 09:11:23AM +0200, Michal Hocko wrote: > > > > > On Thu 01-10-20 21:26:26, Uladzislau Rezki wrote: > > > > > > > > > > > > > > No, I meant going back to idea of new gfp flag, but adjust the implementation in > > > > > > > the allocator (different from what you posted in previous version) so that it > > > > > > > only looks at the flag after it tries to allocate from pcplist and finds out > > > > > > > it's empty. So, no inventing of new page allocator entry points or checks such > > > > > > > as the one you wrote above, but adding the new gfp flag in a way that it doesn't > > > > > > > affect existing fast paths. > > > > > > > > > > > > > OK. Now i see. Please have a look below at the patch, so we fully understand > > > > > > each other. If that is something that is close to your view or not: > > > > > > > > > > > > > > > > > > t a/include/linux/gfp.h b/include/linux/gfp.h > > > > > > index c603237e006c..7e613560a502 100644 > > > > > > --- a/include/linux/gfp.h > > > > > > +++ b/include/linux/gfp.h > > > > > > @@ -39,8 +39,9 @@ struct vm_area_struct; > > > > > > #define ___GFP_HARDWALL 0x100000u > > > > > > #define ___GFP_THISNODE 0x200000u > > > > > > #define ___GFP_ACCOUNT 0x400000u > > > > > > +#define ___GFP_NO_LOCKS 0x800000u > > > > > > > > > > Even if a new gfp flag gains a sufficient traction and support I am > > > > > _strongly_ opposed against consuming another flag for that. Bit space is > > > > > limited. > > > > > > > > That is definitely true. I'm not happy with the GFP flag at all, the > > > > comment is at best a damage limiting move. It still would be better for > > > > a memory pool to be reserved and sized for critical allocations. > > > > > > Completely agreed. The only existing usecase is so special cased that a > > > dedicated pool is not only easier to maintain but it should be also much > > > better tuned for the specific workload. Something not really feasible > > > with the allocator. > > > > > > > > Besides that we certainly do not want to allow craziness like > > > > > __GFP_NO_LOCK | __GFP_RECLAIM (and similar), do we? > > > > > > > > That would deserve to be taken to a dumpster and set on fire. The flag > > > > combination could be checked in the allocator but the allocator path fast > > > > paths are bad enough already. > > > > > > If a new allocation/gfp mode is absolutely necessary then I believe that > > > the most reasoanble way forward would be > > > #define GFP_NO_LOCK ((__force gfp_t)0) > > > > > Agree. Even though i see that some code should be adjusted for it. There are > > a few users of the __get_free_page(0); So, need to double check it: > > Yes, I believe I have pointed that out in the previous discussion. > OK. I spent more time on it. A passed gfp_mask can be adjusted on the entry, that adjustment depends on the gfp_allowed_mask. It can be changed in run-time. For example during early boot it excludes: __GFP_RECLAIM|__GFP_IO|__GFP_FS flags, what is GFP_KERNEL. So, GFP_KERNEL is adjusted on entry and becomes 0 during early boot phase. How to distinguish it: + /* + * gfp_mask can become zero because gfp_allowed_mask changes in run-time. + */ + if (!gfp_mask) + alloc_flags |= ALLOC_NO_LOCKS; + gfp_mask &= gfp_allowed_mask; alloc_mask = gfp_mask; if (!prepare_alloc_pages(gfp_mask, order, preferred_nid, nodemask, &ac, &alloc_mask, &alloc_flags)) > > > > Apart of that. There is a post_alloc_hook(), that gets called from the prep_new_page(). > > If "debug page alloc enabled", it maps a page for debug purposes invoking kernel_map_pages(). > > __kernel_map_pages() is ARCH specific. For example, powerpc variant uses sleep-able locks > > what can be easily converted to raw variant. > > Yes, there are likely more surprises like that. I am not sure about > kasan, page owner (which depens on the stack unwinder) and others which > hook into this path. > I have checked kasan_alloc_pages(), kernel_poison_pages() both are OK, at least i did not find any locking there. As for set_page_owner(), it requires more attention, since it uses arch specific unwind logic. Though, i spent some time on it and so far have not noticed anything. -- Vlad Rezki