Messages in this thread | | | Subject | Re: [PATCH 00/35] Enhance memory utilization with DMEMFS | From | Joao Martins <> | Date | Mon, 19 Oct 2020 20:03:09 +0100 |
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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@oracle.com/
Joao
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