From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:34586 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751266AbdJWKoh (ORCPT ); Mon, 23 Oct 2017 06:44:37 -0400 Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v9NAiVIU077387 for ; Mon, 23 Oct 2017 06:44:37 -0400 Received: from e06smtp13.uk.ibm.com (e06smtp13.uk.ibm.com [195.75.94.109]) by mx0b-001b2d01.pphosted.com with ESMTP id 2dsayhj51k-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 23 Oct 2017 06:44:36 -0400 Received: from localhost by e06smtp13.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 23 Oct 2017 11:44:34 +0100 Date: Mon, 23 Oct 2017 12:44:27 +0200 From: Martin Schwidefsky Subject: Re: [PATCH v3 02/13] dax: require 'struct page' for filesystem dax In-Reply-To: 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> <20171023071835.67ee5210@mschwideX1> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Message-Id: <20171023124427.10d15ee3@mschwideX1> Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: Dan Williams Cc: Christoph Hellwig , 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 Mon, 23 Oct 2017 01:55:20 -0700 Dan Williams wrote: > On Sun, Oct 22, 2017 at 10:18 PM, Martin Schwidefsky > wrote: > > 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. > > For read-only memory I think we can enable a subset of DAX, and > explicitly turn off the paths that require get_user_pages(). However, > I wonder if anyone has tested DAX with dcssblk because fork() requires > get_user_pages()? I did not test it recently, someone else might have. Gerald? Looking at the code I see this in the s390 version of gup_pte_range: mask = (write ? _PAGE_PROTECT : 0) | _PAGE_INVALID | _PAGE_SPECIAL; ... if ((pte_val(pte) & mask) != 0) return 0; ... The XIP code used the pte_mkspecial mechanics to make it work. As far as I can see the pfn_t_devmap returns true for the DAX mappins, yes? Then I would say that dcssblk and DAX currently do not work together. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.