From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 23 Oct 2017 12:44:27 +0200 From: Martin Schwidefsky 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 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: quoted-printable Message-Id: <20171023124427.10d15ee3@mschwideX1> Sender: owner-linux-mm@kvack.org List-ID: 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: > > =20 > >> On Fri, Oct 20, 2017 at 08:23:02AM -0700, Dan Williams wrote: =20 > >> > 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 DA= X, > >> > 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() f= or > >> > just that case. =20 > >> > >> 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 numb= er. > >> 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. =20 > > > > 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. =20 >=20 > 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()? =20 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 =3D (write ? _PAGE_PROTECT : 0) | _PAGE_INVALID | _PAGE_SPECIA= L; ... if ((pte_val(pte) & mask) !=3D 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. --=20 blue skies, Martin. "Reality continues to ruin my life." - Calvin. -- 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