According to the following documentationhttps://www.kernel.org/doc/html/latest/x86/pat.html,
Drivers wanting to export some pages to userspace do it by using mmap interface and a combination of:
pgprot_noncached()
io_remap_pfn_range() or remap_pfn_range() or vmf_insert_pfn()
Note that this set of APIs only works with IO (non RAM) regions. If driver wants to export a RAM region, it has to do set_memory_uc() or set_memory_wc() as step 0 above and also track the usage of those pages and use set_memory_wb() before the page is freed to free pool.
Why is the extra step set_memory_uc() or set_memory_wc() needed for RAM regions?