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[195.135.220.15]) by mx.google.com with ESMTPS id k30si1941854edb.244.2019.03.14.06.29.34 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 14 Mar 2019 06:29:34 -0700 (PDT) Received-SPF: pass (google.com: domain of mhocko@suse.com designates 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=pass (google.com: domain of mhocko@suse.com designates 195.135.220.15 as permitted sender) smtp.mailfrom=mhocko@suse.com X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 00EEDAFBB; Thu, 14 Mar 2019 13:29:34 +0000 (UTC) Date: Thu, 14 Mar 2019 14:29:33 +0100 From: Michal Hocko To: Takashi Iwai Cc: Vlastimil Babka , linux-mm@kvack.org, "Kirill A. Shutemov" , Mel Gorman Subject: Re: [PATCH v2] mm, page_alloc: disallow __GFP_COMP in alloc_pages_exact() Message-ID: <20190314132933.GL7473@dhcp22.suse.cz> References: <20190314093944.19406-1-vbabka@suse.cz> <20190314094249.19606-1-vbabka@suse.cz> <20190314101526.GH7473@dhcp22.suse.cz> <1dc997a3-7573-7bd5-9ce6-3bfbf77d1194@suse.cz> <20190314113626.GJ7473@dhcp22.suse.cz> <20190314120939.GK7473@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) 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 Thu 14-03-19 14:15:38, Takashi Iwai wrote: > On Thu, 14 Mar 2019 13:09:39 +0100, > Michal Hocko wrote: > > > > On Thu 14-03-19 12:56:43, Takashi Iwai wrote: > > > On Thu, 14 Mar 2019 12:36:26 +0100, > > > Michal Hocko wrote: > > > > > > > > On Thu 14-03-19 11:30:03, Vlastimil Babka wrote: [...] > > > > > I initially went with 2 as well, as you can see from v1 :) but then I looked at > > > > > the commit [2] mentioned in [1] and I think ALSA legitimaly uses __GFP_COMP so > > > > > that the pages are then mapped to userspace. Breaking that didn't seem good. > > > > > > > > It used the flag legitimately before because they were allocating > > > > compound pages but now they don't so this is just a conversion bug. > > > > > > We still use __GFP_COMP for allocation of the sound buffers that are > > > also mmapped to user-space. The mentioned commit above [2] was > > > reverted later. > > > > Yes, I understand that part. __GFP_COMP makes sense on a comound page. > > But if you are using alloc_pages_exact then the flag doesn't make sense > > because split out should already do what you want. Unless I am missing > > something. > > The __GFP_COMP was taken as a sort of workaround for the problem wrt > mmap I already forgot. If it can be eliminated, it's all good. Without __GFP_COMP you would get tail pages which are not setup properly AFAIU. With alloc_pages_exact you should get an "array" of head pages which are properly reference counted. But I might misunderstood the original problem which __GFP_COMP tried to solve. -- Michal Hocko SUSE Labs