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

[yulei zhang]

On Tue, Oct 20, 2020 at 3:03 AM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote:
>
> 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

Just as Joao mentioned, remap_pfn_range() may request a single
contiguous PFN range, which
is not our intention. And for VM_DMEM, I think we may drop it in the
next version, and to use the
existing bits as much as possible to minimize the modifications.