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

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Date:   Wed, 14 Oct 2020 15:25:25 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     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>,
        Paolo Bonzini <pbonzini@...hat.com>,
        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 Mon, Oct 12, 2020 at 4:00 AM Joao Martins <joao.m.martins@...cle.com> wrote:
[..]
> On 10/10/20 9:15 AM, yulei zhang wrote:
> > On Fri, Oct 9, 2020 at 7:53 PM Joao Martins <joao.m.martins@...cle.com> wrote:
> >> On 10/9/20 12:39 PM, yulei zhang wrote:
> >>> Joao, thanks a lot for the feedback. One more thing needs to mention
> >>> is that dmemfs also support fine-grained
> >>> memory management which makes it more flexible for tenants with
> >>> different requirements.
> >>>
> >> So as DAX when it allows to partition a region (starting 5.10). Meaning you have a region
> >> which you dedicated to userspace. That region can then be partitioning into devices which
> >> give you access to multiple (possibly discontinuous) extents with at a given page
> >> granularity (selectable when you create the device), accessed through mmap().
> >> You can then give that device to a cgroup. Or you can return that memory back to the
> >> kernel (should you run into OOM situation), or you recreate the same mappings across
> >> reboot/kexec.
> >>
> >> I probably need to read your patches again, but can you extend on the 'dmemfs also support
> >> fine-grained memory management' to understand what is the gap that you mention?
> >>
> > sure, dmemfs uses bitmap to track the memory usage in the reserved
> > memory region in
> > a given page size granularity. And for each user the memory can be
> > discrete as well.
> >
> That same functionality of tracking reserved region usage across different users at any
> page granularity is covered the DAX series I mentioned below. The discrete part -- IIUC
> what you meant -- is then reduced using DAX ABI/tools to create a device file vs a filesystem.

Put another way. Linux already has a fine grained memory management
system, the page allocator. 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?