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

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Date:   Mon, 19 Oct 2020 15:37:51 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Joao Martins <joao.m.martins@...cle.com>
Cc:     yulei zhang <yulei.kernel@...il.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Haiwei Li <lihaiwei.kernel@...il.com>,
        Yulei Zhang <yuleixzhang@...cent.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Jane Y Chu <jane.chu@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH 00/35] Enhance memory utilization with DMEMFS

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