linux-kernel - Re: [PATCH v3 02/13] dax: require 'struct page' for filesystem dax

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Message-Id: <20171023071835.67ee5210@mschwideX1>
Date:   Mon, 23 Oct 2017 07:18:35 +0200
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Jeff Moyer <jmoyer@...hat.com>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>
Subject: Re: [PATCH v3 02/13] dax: require 'struct page' for filesystem dax

On Fri, 20 Oct 2017 18:29:33 +0200
Christoph Hellwig <hch@....de> 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.