From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qa0-f49.google.com (mail-qa0-f49.google.com [209.85.216.49]) by kanga.kvack.org (Postfix) with ESMTP id 033E26B0100 for ; Thu, 12 Jun 2014 05:44:21 -0400 (EDT) Received: by mail-qa0-f49.google.com with SMTP id w8so1269019qac.36 for ; Thu, 12 Jun 2014 02:44:21 -0700 (PDT) Received: from mail-qc0-x234.google.com (mail-qc0-x234.google.com [2607:f8b0:400d:c01::234]) by mx.google.com with ESMTPS id o9si446672qac.99.2014.06.12.02.44.21 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 12 Jun 2014 02:44:21 -0700 (PDT) Received: by mail-qc0-f180.google.com with SMTP id i17so1490184qcy.25 for ; Thu, 12 Jun 2014 02:44:21 -0700 (PDT) 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 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Andrew Morton , Linus Torvalds , Linux Kernel , linux-mm , Peter Zijlstra , Ingo Molnar , Linux API 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 mappi= ng, >> > 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-tri= vial >> > code in kernel virtual memory subsystem including hot paths. Also to g= et >> > nonlinear mapping work kernel need a way to distinguish normal page ta= ble >> > 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 wil= d. >> > It's only known that one enterprise RDBMS implementation uses the sysc= all >> > on 32-bit systems to map files bigger than can linearly fit into 32-bi= t >> > 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=E2= =80=90 >> sider migrating to alternatives. >> >> Okay? > > Yep. Looks okay to me. Thanks for checking. Cheers, Michael --=20 Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org