Re: [PATCH 00/35] Enhance memory utilization with DMEMFS

[Paolo Bonzini]

On 15/10/20 00:25, Dan Williams wrote:
> Now, with recent device-dax extensions, it
> also has a coarse grained memory management system for  physical
> address-space partitioning and a path for struct-page-less backing for
> VMs. What feature gaps remain vs dmemfs, and can those gaps be closed
> with incremental improvements to the 2 existing memory-management
> systems?

If I understand correctly, devm_memremap_pages() on ZONE_DEVICE memory
would still create the "struct page" albeit lazily?  KVM then would use
the usual get_user_pages() path.

Looking more closely at the implementation of dmemfs, I don't understand
is why dmemfs needs VM_DMEM etc. and cannot provide access to mmap-ed
memory using remap_pfn_range and VM_PFNMAP, just like /dev/mem.  If it
did that KVM would get physical addresses using fixup_user_fault and
never need pfn_to_page() or get_user_pages().  I'm not saying that would
instantly be an approval, but it would make remove a lot of hooks.

Paolo