From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:38890 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750756AbdJWFSq (ORCPT ); Mon, 23 Oct 2017 01:18:46 -0400 Received: from pps.filterd (m0098396.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v9N5EY9N095409 for ; Mon, 23 Oct 2017 01:18:46 -0400 Received: from e06smtp12.uk.ibm.com (e06smtp12.uk.ibm.com [195.75.94.108]) by mx0a-001b2d01.pphosted.com with ESMTP id 2drw0cq2ay-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 23 Oct 2017 01:18:45 -0400 Received: from localhost by e06smtp12.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 23 Oct 2017 06:18:43 +0100 Date: Mon, 23 Oct 2017 07:18:35 +0200 From: Martin Schwidefsky Subject: Re: [PATCH v3 02/13] dax: require 'struct page' for filesystem dax In-Reply-To: <20171020162933.GA26320@lst.de> References: <150846713528.24336.4459262264611579791.stgit@dwillia2-desk3.amr.corp.intel.com> <150846714747.24336.14704246566580871364.stgit@dwillia2-desk3.amr.corp.intel.com> <20171020075735.GA14378@lst.de> <20171020162933.GA26320@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Message-Id: <20171023071835.67ee5210@mschwideX1> Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: Christoph Hellwig Cc: Dan Williams , Andrew Morton , Jan Kara , "linux-nvdimm@lists.01.org" , Benjamin Herrenschmidt , Heiko Carstens , "linux-kernel@vger.kernel.org" , linux-xfs@vger.kernel.org, Linux MM , Jeff Moyer , Paul Mackerras , Michael Ellerman , linux-fsdevel , Ross Zwisler , Gerald Schaefer On Fri, 20 Oct 2017 18:29:33 +0200 Christoph Hellwig wrote: > On Fri, Oct 20, 2017 at 08:23:02AM -0700, Dan Williams wrote: > > Yes, however it seems these drivers / platforms have been living with > > the lack of struct page for a long time. So they either don't use DAX, > > or they have a constrained use case that never triggers > > get_user_pages(). If it is the latter then they could introduce a new > > configuration option that bypasses the pfn_t_devmap() check in > > bdev_dax_supported() and fix up the get_user_pages() paths to fail. > > So, I'd like to understand how these drivers have been using DAX > > support without struct page to see if we need a workaround or we can > > go ahead delete this support. If the usage is limited to > > execute-in-place perhaps we can do a constrained ->direct_access() for > > just that case. > > For axonram I doubt anyone is using it any more - it was a very for > the IBM Cell blades, which were produceѕ in a rather limited number. > And Cell basically seems to be dead as far as I can tell. > > For S/390 Martin might be able to help out what the status of xpram > in general and DAX support in particular is. The goes back to the time where DAX was called XIP. The initial design point has been *not* to have struct pages for a large read-only memory area. There is a block device driver for z/VM that maps a DCSS segment somewhere in memore (no struct page!) with e.g. the complete /usr filesystem. The xpram driver is a different beast and has nothing to do with XIP/DAX. Now, if any there are very few users of the dcssblk driver out there. The idea to save a few megabyte for /usr never really took of. We have to look at our get_user_pages() implementation to see how hard it would be to make it fail if the target address is for an area without struct pages. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.