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Subject: Re: [PATCH 00/35] Enhance memory utilization with DMEMFS
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>
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        Wanpeng Li <kernellwp@gmail.com>,
        Haiwei Li <lihaiwei.kernel@gmail.com>,
        Yulei Zhang <yuleixzhang@tencent.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Jane Y Chu <jane.chu@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
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From:   Joao Martins <joao.m.martins@oracle.com>
Message-ID: <0c8fd8ab-c0d4-003a-6943-1ec732c96e1c@oracle.com>
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