Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933082AbaFLJo0 (ORCPT ); Thu, 12 Jun 2014 05:44:26 -0400 Received: from mail-qa0-f48.google.com ([209.85.216.48]:43947 "EHLO mail-qa0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932740AbaFLJoW convert rfc822-to-8bit (ORCPT ); Thu, 12 Jun 2014 05:44:22 -0400 MIME-Version: 1.0 Reply-To: mtk.manpages@gmail.com In-Reply-To: <20140612094014.BFEA4E00A2@blue.fi.intel.com> References: <1399552888-11024-1-git-send-email-kirill.shutemov@linux.intel.com> <1399552888-11024-2-git-send-email-kirill.shutemov@linux.intel.com> <20140612094014.BFEA4E00A2@blue.fi.intel.com> From: "Michael Kerrisk (man-pages)" Date: Thu, 12 Jun 2014 11:44:01 +0200 Message-ID: Subject: Re: [PATCH 1/2] mm: mark remap_file_pages() syscall as deprecated To: "Kirill A. Shutemov" Cc: Andrew Morton , Linus Torvalds , Linux Kernel , linux-mm , Peter Zijlstra , Ingo Molnar , Linux API Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Kirill, On Thu, Jun 12, 2014 at 11:40 AM, Kirill A. Shutemov wrote: > Michael Kerrisk wrote: >> Hi Kirill, >> >> On Thu, May 8, 2014 at 2:41 PM, Kirill A. Shutemov >> wrote: >> > The remap_file_pages() system call is used to create a nonlinear mapping, >> > that is, a mapping in which the pages of the file are mapped into a >> > nonsequential order in memory. The advantage of using remap_file_pages() >> > over using repeated calls to mmap(2) is that the former approach does not >> > require the kernel to create additional VMA (Virtual Memory Area) data >> > structures. >> > >> > Supporting of nonlinear mapping requires significant amount of non-trivial >> > code in kernel virtual memory subsystem including hot paths. Also to get >> > nonlinear mapping work kernel need a way to distinguish normal page table >> > entries from entries with file offset (pte_file). Kernel reserves flag in >> > PTE for this purpose. PTE flags are scarce resource especially on some CPU >> > architectures. It would be nice to free up the flag for other usage. >> > >> > Fortunately, there are not many users of remap_file_pages() in the wild. >> > It's only known that one enterprise RDBMS implementation uses the syscall >> > on 32-bit systems to map files bigger than can linearly fit into 32-bit >> > virtual address space. This use-case is not critical anymore since 64-bit >> > systems are widely available. >> > >> > The plan is to deprecate the syscall and replace it with an emulation. >> > The emulation will create new VMAs instead of nonlinear mappings. It's >> > going to work slower for rare users of remap_file_pages() but ABI is >> > preserved. >> > >> > One side effect of emulation (apart from performance) is that user can hit >> > vm.max_map_count limit more easily due to additional VMAs. See comment for >> > DEFAULT_MAX_MAP_COUNT for more details on the limit. >> >> Best to CC linux-api@ >> (https://www.kernel.org/doc/man-pages/linux-api-ml.html) on patches >> like this, as well as the man-pages maintainer, so that something goes >> into the man page. I added the following into the man page: >> >> Note: this system call is (since Linux 3.16) deprecated and >> will eventually be replaced by a slower in-kernel emulation. >> Those few applications that use this system call should con‐ >> sider migrating to alternatives. >> >> Okay? > > Yep. Looks okay to me. Thanks for checking. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/