On Fri, 20 Oct 2017 18:29:33 +0200
Christoph Hellwig <hch(a)lst.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
"Reality continues to ruin my life." - Calvin.