On Mon, Oct 23, 2017 at 3:44 AM, Martin Schwidefsky <schwidefsky@xxxxxxxxxx> wrote: > 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 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()? > > 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? Yes, but that's only for get_user_pages_fast() support. > Then I would say that dcssblk and DAX currently do not work together. I think at a minimum we need a new pfn_t flag for the 'special' bit to at least indicate that DAX mappings of dcssblk and axonram do not support normal get_user_pages(). Then I don't need to explicitly disable DAX in the !pfn_t_devmap() case. I think I also want to split the "pfn_to_virt()" and the "sector to pfn" operations into distinct dax_operations rather than doing both in one ->direct_access(). This supports storing pfns in the fs/dax radix rather than sectors. In other words, the pfn_t_devmap() requirement was only about making get_user_pages() safely fail, and pte_special() fills that requirement.
