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 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. 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()?
