Re: [PATCH] mm/page_table_check: Fix crash on ZONE_DEVICE

[Dan Williams <dan.j.williams@intel.com>]

[ add Alistair ]

Peter Xu wrote:
> Not all pages may apply to pgtable check.  One example is ZONE_DEVICE
> pages: they map PFNs directly, and they don't allocate page_ext at all even
> if there's struct page around.  One may reference devm_memremap_pages().
> 
> When both ZONE_DEVICE and page-table-check enabled, then try to map some
> dax memories, one can trigger kernel bug constantly now when the kernel was
> trying to inject some pfn maps on the dax device:
> 
>  kernel BUG at mm/page_table_check.c:55!
> 
> While it's pretty legal to use set_pxx_at() for ZONE_DEVICE pages for page
> fault resolutions, skip all the checks if page_ext doesn't even exist in
> pgtable checker, which applies to ZONE_DEVICE but maybe more.

This looks correct to me, and needed in the near term. You can add:

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

In the long term, the page_ext check may not be needed. I.e. the reason
I added Alistair was in case his work to make DAX pages behave like
typical pages for reference counting would also make them behave the
same for the presence of page_ext.