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30 Oct 2019 10:58:49 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.68,248,1569308400"; d="scan'208";a="203954492" Received: from ray.jf.intel.com (HELO [10.7.201.139]) ([10.7.201.139]) by orsmga006.jf.intel.com with ESMTP; 30 Oct 2019 10:58:49 -0700 Subject: Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings To: "Edgecombe, Rick P" , "peterz@infradead.org" Cc: "adobriyan@gmail.com" , "linux-kernel@vger.kernel.org" , "rppt@kernel.org" , "rostedt@goodmis.org" , "jejb@linux.ibm.com" , "tglx@linutronix.de" , "linux-mm@kvack.org" , "dave.hansen@linux.intel.com" , "linux-api@vger.kernel.org" , "x86@kernel.org" , "akpm@linux-foundation.org" , "hpa@zytor.com" , "mingo@redhat.com" , "luto@kernel.org" , "kirill@shutemov.name" , "bp@alien8.de" , "rppt@linux.ibm.com" , "arnd@arndb.de" References: <1572171452-7958-1-git-send-email-rppt@kernel.org> <1572171452-7958-2-git-send-email-rppt@kernel.org> <20191028123124.ogkk5ogjlamvwc2s@box> <20191028130018.GA7192@rapoport-lnx> <20191028131623.zwuwguhm4v4s5imh@box> <20191028135521.GB4097@hirez.programming.kicks-ass.net> <0a35765f7412937c1775daa05177b20113760aee.camel@intel.com> <20191028210052.GM4643@worktop.programming.kicks-ass.net> <69c57f7fa9a1be145827673b37beff155a3adc3c.camel@intel.com> <20191030100418.GV4097@hirez.programming.kicks-ass.net> From: Dave Hansen Openpgp: preference=signencrypt Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US 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 10/30/19 10:48 AM, Edgecombe, Rick P wrote: > On Wed, 2019-10-30 at 11:04 +0100, Peter Zijlstra wrote: >>> You mean shatter performance? >> >> Shatter (all) large pages. >=20 > So it looks like this is already happening then to some degree. It's no= t just > BPF either, any module_alloc() user is going to do something similar wi= th the > direct map alias of the page they got for the text. >=20 > So there must be at least some usages where breaking the direct map dow= n, for > like a page to store a key or something, isn't totally horrible. The systems that really need large pages are the large ones. They have the same TLBs and data structures as really little systems, but orders of magnitude more address space. Modules and BPF are a (hopefully) drop in the bucket on small systems and they're really inconsequential on really big systems. Modules also require privilege. Allowing random user apps to fracture the direct map for every page of their memory or *lots* of pages of their memory is an entirely different kind of problem from modules. It takes a "drop in the bucket" fracturing and turns it into the common case.