On 17.06.25 17:43, David Hildenbrand wrote:
RFC because it's based on mm-new where some things might still change around the devmap removal stuff. While removing support for CoW PFNMAPs is a noble goal, I am not even sure if we can remove said support for e.g., /dev/mem that easily. In the end, Cow PFNMAPs are pretty simple: everything is "special" except CoW'ed anon folios, that are "normal". The only complication is: how to identify such pages without pte_special(). Because with pte_special(), it's easy. Well, of course, one day all architectures might support pte_special() ... either because we added support for pte_special() or removed support for ... these architectures from Linux. No need to wait for that day. Let's do some cleanups around vm_normal_page()/vm_normal_page_pmd() and handling of the huge zero folio, and remove the "horrible special case to handle copy-on-write behaviour" that does questionable things in remap_pfn_range() with a VMA, simply by ... looking for anonymous folios in CoW PFNMAPs to identify anonymous folios? I know, sounds crazy ;)
I'll mention one corner case that just occurred to me: assume someone maps arbitrary /dev/mem that is actually used by the kernel for user space, and then some of that memory gets allocated as anonymous memory, it would probably be a problem.
Hmm, I'll have to think about that, and the interaction with CONFIG_STRICT_DEVMEM.
-- Cheers, David / dhildenb
