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

[Dan Williams]

On Mon, Oct 12, 2020 at 4:00 AM Joao Martins <joao.m.martins@xxxxxxxxxx> 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@xxxxxxxxxx> 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?