Re: [RFC V2 00/37] Enhance memory utilization with DMEMFS

[Dan Williams]

On Mon, Dec 7, 2020 at 4:03 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 07.12.20 12:30, yulei.kernel@xxxxxxxxx wrote:
> > From: Yulei Zhang <yuleixzhang@xxxxxxxxxxx>
> >
> > In current system each physical memory page is assocaited with
> > a page structure which is used to track the usage of this page.
> > But due to the memory usage rapidly growing in cloud environment,
> > we find the resource consuming for page structure storage becomes
> > more and more remarkable. So is it possible that we could reclaim
> > such memory and make it reusable?
> >
> > This patchset introduces an idea about how to save the extra
> > memory through a new virtual filesystem -- dmemfs.
> >
> > Dmemfs (Direct Memory filesystem) is device memory or reserved
> > memory based filesystem. This kind of memory is special as it
> > is not managed by kernel and most important it is without 'struct page'.
> > Therefore we can leverage the extra memory from the host system
> > to support more tenants in our cloud service.
>
> "is not managed by kernel" well, it's obviously is managed by the
> kernel. It's not managed by the buddy ;)
>
> How is this different to using "mem=X" and mapping the relevant memory
> directly into applications? Is this "simply" a control instance on top
> that makes sure unprivileged process can access it and not step onto
> each others feet? Is that the reason why it's called  a "file system"?
> (an example would have helped here, showing how it's used)
>
> It's worth noting that memory hotunplug, memory poisoning and probably
> more is currently fundamentally incompatible with this approach - which
> should better be pointed out in the cover letter.
>
> Also, I think something similar can be obtained by using dax/hmat
> infrastructure with "memmap=", at least I remember a talk where this was
> discussed (but not sure if they modified the firmware to expose selected
> memory as soft-reserved - we would only need a cmdline parameter to
> achieve the same - Dan might know more).

There is currently the efi_fake_mem parameter that can add the
"EFI_MEMORY_SP" attribute on EFI platforms:

    efi_fake_mem=4G@9G:0x40000

...this results in a /dev/dax instance that can be further partitioned
via the device-dax sub-division facility merged for 5.10. That could
be generalized to something else for non-EFI platforms, but there has
not been a justification to go that route yet.

Joao pointed this out in a previous posting of DMEMFS, and I have yet
to see an explanation of incremental benefit the kernel gains from
having yet another parallel memory management interface.