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

[Joao Martins]

On 10/19/20 2:37 PM, Paolo Bonzini wrote:
> 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.
> 
Correct.

The removal of struct page would be one of the added incremental improvements, like a
'map' with 'raw' sysfs attribute for dynamic dax regions that wouldn't online/create the
struct pages. The remaining plumbing (...)

> 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.
> 

(...) is similar to what you describe above. Albeit there's probably no need to do a
remap_pfn_range at mmap(), as DAX supplies a fault/huge_fault. Also, using that means it's
limited to a single contiguous PFN chunk.

KVM has the bits to make it work without struct pages, I don't think there's a need for
new pg/pfn_t/VM_* bits (aside from relying on {PFN,PAGE}_SPECIAL) as mentioned at the
start of the thread. I'm storing my wip here:

	https://github.com/jpemartins/linux pageless-dax

Which is based on the first series that had been submitted earlier this year:

	https://lore.kernel.org/kvm/20200110190313.17144-1-joao.m.martins@xxxxxxxxxx/

  Joao