Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754035AbbDMKpi (ORCPT ); Mon, 13 Apr 2015 06:45:38 -0400 Received: from mail-wi0-f180.google.com ([209.85.212.180]:37244 "EHLO mail-wi0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753357AbbDMKph (ORCPT ); Mon, 13 Apr 2015 06:45:37 -0400 Date: Mon, 13 Apr 2015 12:45:32 +0200 From: Ingo Molnar To: Christoph Hellwig Cc: Linus Torvalds , linux-kernel@vger.kernel.org, linux-nvdimm@ml01.01.org, Ross Zwisler , Dan Williams , Boaz Harrosh , Matthew Wilcox Subject: Re: [GIT PULL] PMEM driver for v4.1 Message-ID: <20150413104531.GB30556@gmail.com> References: <20150413093309.GA30219@gmail.com> <20150413093541.GA5147@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150413093541.GA5147@lst.de> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1496 Lines: 39 * Christoph Hellwig wrote: > On Mon, Apr 13, 2015 at 11:33:09AM +0200, Ingo Molnar wrote: > > Limitations: this is a regular block device, and since the pmem areas > > are not struct page backed, they are invisible to the rest of the > > system (other than the block IO device), so direct IO to/from pmem > > areas, direct mmap() or XIP is not possible yet. The page cache will > > also shadow and double buffer pmem contents, etc. > > Unless you use the DAX support in ext2/4 and soon XFS, in which case > we avoid that double buffering when doing read/write and mmap Indeed, I missed that DAX support just went upstream in v4.0 - nice! DAX may have some other limitations though that comes from not having struct page * backing and using VM_MIXEDMAP, the following APIs might not work on DAX files: - splice - zero copy O_DIRECT into DAX areas. - futexes - ( AFAICS hugetlbs won't work on DAX mmap()s yet - although with the current nocache mapping that's probable the least of the performance issues for now. ) Btw., what's the future design plan here? Enable struct page backing, or provide special codepaths for all DAX uses like the special pte based approach for mmap()s? Thanks, Ingo -- 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/