From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f66.google.com ([209.85.218.66]:43547 "EHLO mail-oi0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752041AbdJTW36 (ORCPT ); Fri, 20 Oct 2017 18:29:58 -0400 Received: by mail-oi0-f66.google.com with SMTP id c77so22475465oig.0 for ; Fri, 20 Oct 2017 15:29:58 -0700 (PDT) MIME-Version: 1.0 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> From: Dan Williams Date: Fri, 20 Oct 2017 15:29:57 -0700 Message-ID: Subject: Re: [PATCH v3 02/13] dax: require 'struct page' for filesystem dax Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: Christoph Hellwig Cc: 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 , Martin Schwidefsky , linux-fsdevel , Ross Zwisler , Gerald Schaefer On Fri, Oct 20, 2017 at 9:29 AM, 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 produce=D1=95 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. Ok, I'd also like to kill DAX support in the brd driver. It's a source of complexity and maintenance burden for zero benefit. It's the only ->direct_access() implementation that sleeps and it's the only implementation where there is a non-linear relationship between sectors and pfns. Having a 1:1 sector to pfn relationship will help with the dma-extent-busy management since we don't need to keep calling into the driver to map pfns back to sectors once we know the pfn[0] sector[0] relationship.