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

[Martin Schwidefsky]

On Mon, 23 Oct 2017 01:55:20 -0700
Dan Williams <dan.j.williams@xxxxxxxxx> wrote:

> On Sun, Oct 22, 2017 at 10:18 PM, Martin Schwidefsky
> <schwidefsky@xxxxxxxxxx> wrote:
> > On Fri, 20 Oct 2017 18:29:33 +0200
> > Christoph Hellwig <hch@xxxxxx> 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 produceÑ 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.