From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752158AbbHMM52 (ORCPT ); Thu, 13 Aug 2015 08:57:28 -0400 Received: from mail-wi0-f180.google.com ([209.85.212.180]:37511 "EHLO mail-wi0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751460AbbHMM51 (ORCPT ); Thu, 13 Aug 2015 08:57:27 -0400 MIME-Version: 1.0 In-Reply-To: <55CC3222.5090503@plexistor.com> References: <20150813025112.36703.21333.stgit@otcpl-skl-sds-2.jf.intel.com> <20150813030109.36703.21738.stgit@otcpl-skl-sds-2.jf.intel.com> <55CC3222.5090503@plexistor.com> Date: Thu, 13 Aug 2015 05:57:26 -0700 Message-ID: Subject: Re: [PATCH v5 2/5] allow mapping page-less memremaped areas into KVA From: Dan Williams To: Boaz Harrosh Cc: "linux-kernel@vger.kernel.org" , Jens Axboe , Rik van Riel , "linux-nvdimm@lists.01.org" , Linux MM , Mel Gorman , "torvalds@linux-foundation.org" , Christoph Hellwig Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 12, 2015 at 10:58 PM, Boaz Harrosh wrote: > On 08/13/2015 06:01 AM, Dan Williams wrote: [..] >> +void *kmap_atomic_pfn_t(__pfn_t pfn) >> +{ >> + struct page *page = __pfn_t_to_page(pfn); >> + resource_size_t addr; >> + struct kmap *kmap; >> + >> + rcu_read_lock(); >> + if (page) >> + return kmap_atomic(page); > > Right even with pages I pay rcu_read_lock(); for every access? > >> + addr = __pfn_t_to_phys(pfn); >> + list_for_each_entry_rcu(kmap, &ranges, list) >> + if (addr >= kmap->res->start && addr <= kmap->res->end) >> + return kmap->base + addr - kmap->res->start; >> + > > Good god! This loop is a real *joke*. You have just dropped memory access > performance by 10 fold. > > The all point of pages and memory_model.h was to have a one to one > relation-ships between Kernel-virtual vs physical vs page * > > There is already an object that holds a relationship of physical > to Kernel-virtual. It is called a memory-section. Why not just > widen its definition? > > If you are willing to accept this loop. In current Linux 2015 Kernel > Then I have nothing farther to say. > > Boaz - go mourning for the death of the Linux Kernel alone in the corner ;-( > This is explicitly addressed in the changelog, repeated here: > The __pfn_t to resource lookup is indeed inefficient walking of a linked list, > but there are two mitigating factors: > > 1/ The number of persistent memory ranges is bounded by the number of > DIMMs which is on the order of 10s of DIMMs, not hundreds. > > 2/ The lookup yields the entire range, if it becomes inefficient to do a > kmap_atomic_pfn_t() a PAGE_SIZE at a time the caller can take > advantage of the fact that the lookup can be amortized for all kmap > operations it needs to perform in a given range. DAX as is is races against pmem unbind. A synchronization cost must be paid somewhere to make sure the memremap() mapping is still valid.