From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755929AbaEHV5b (ORCPT ); Thu, 8 May 2014 17:57:31 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:41921 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753994AbaEHV5a (ORCPT ); Thu, 8 May 2014 17:57:30 -0400 Date: Thu, 8 May 2014 14:57:29 -0700 From: Andrew Morton To: "Kirill A. Shutemov" Cc: Linus Torvalds , linux-kernel@vger.kernel.org, linux-mm@kvack.org, peterz@infradead.org, mingo@kernel.org Subject: Re: [PATCH 2/2] mm: replace remap_file_pages() syscall with emulation Message-Id: <20140508145729.3d82d2c989cfc483c94eb324@linux-foundation.org> In-Reply-To: <1399552888-11024-3-git-send-email-kirill.shutemov@linux.intel.com> References: <1399552888-11024-1-git-send-email-kirill.shutemov@linux.intel.com> <1399552888-11024-3-git-send-email-kirill.shutemov@linux.intel.com> X-Mailer: Sylpheed 3.2.0beta5 (GTK+ 2.24.10; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 8 May 2014 15:41:28 +0300 "Kirill A. Shutemov" wrote: > remap_file_pages(2) was invented to be able efficiently map parts of > huge file into limited 32-bit virtual address space such as in database > workloads. > > Nonlinear mappings are pain to support and it seems there's no > legitimate use-cases nowadays since 64-bit systems are widely available. > > Let's drop it and get rid of all these special-cased code. > > The patch replaces the syscall with emulation which creates new VMA on > each remap_file_pages(), unless they it can be merged with an adjacent > one. > > I didn't find *any* real code that uses remap_file_pages(2) to test > emulation impact on. I've checked Debian code search and source of all > packages in ALT Linux. No real users: libc wrappers, mentions in strace, > gdb, valgrind and this kind of stuff. > > There are few basic tests in LTP for the syscall. They work just fine > with emulation. > > To test performance impact, I've written small test case which > demonstrate pretty much worst case scenario: map 4G shmfs file, write to > begin of every page pgoff of the page, remap pages in reverse order, > read every page. > > The test creates 1 million of VMAs if emulation is in use, so I had to > set vm.max_map_count to 1100000 to avoid -ENOMEM. > > Before: 23.3 ( +- 4.31% ) seconds > After: 43.9 ( +- 0.85% ) seconds > Slowdown: 1.88x > > I believe we can live with that. > There's still all the special-case goop around the place to be cleaned up - VM_NONLINEAR is a decent search term. As is "grep nonlinear mm/*.c". And although this cleanup is the main reason for the patchset, let's not do it now - we can do all that if/after this patch get merged. I'll queue the patches for some linux-next exposure and shall send [1/2] Linuswards for 3.16 if nothing terrible happens. Once we've sorted out the too-many-vmas issue we'll need to work out when to merge [2/2].