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

On Fri, 20 Oct 2017 18:29:33 +0200 Christoph Hellwig <hch(a)lst.de&gt; 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.