Re: [PATCH 0/3] mm: use memmap_on_memory semantics for dax/kmem

[David Hildenbrand]

On 13.07.23 21:12, Jeff Moyer wrote:
> David Hildenbrand <david@xxxxxxxxxx> writes:
>
> > On 16.06.23 00:00, Vishal Verma wrote:
> > > The dax/kmem driver can potentially hot-add large amounts of memory
> > > originating from CXL memory expanders, or NVDIMMs, or other 'device
> > > memories'. There is a chance there isn't enough regular system memory
> > > available to fit ythe memmap for this new memory. It's therefore
> > > desirable, if all other conditions are met, for the kmem managed memory
> > > to place its memmap on the newly added memory itself.
> > >
> > > Arrange for this by first allowing for a module parameter override for
> > > the mhp_supports_memmap_on_memory() test using a flag, adjusting the
> > > only other caller of this interface in dirvers/acpi/acpi_memoryhotplug.c,
> > > exporting the symbol so it can be called by kmem.c, and finally changing
> > > the kmem driver to add_memory() in chunks of memory_block_size_bytes().
> >
> > 1) Why is the override a requirement here? Just let the admin
> > configure it then then add conditional support for kmem.
> >
> > 2) I recall that there are cases where we don't want the memmap to
> > land on slow memory (which online_movable would achieve). Just imagine
> > the slow PMEM case. So this might need another configuration knob on
> > the kmem side.
>
>  From my memory, the case where you don't want the memmap to land on
> *persistent memory* is when the device is small (such as NVDIMM-N), and
> you want to reserve as much space as possible for the application data.
> This has nothing to do with the speed of access.

Now that you mention it, I also do remember the origin of the altmap --
to achieve exactly that: place the memmap on the device.

commit 4b94ffdc4163bae1ec73b6e977ffb7a7da3d06d3
Author: Dan Williams <dan.j.williams@xxxxxxxxx>
Date:   Fri Jan 15 16:56:22 2016 -0800

    x86, mm: introduce vmem_altmap to augment vmemmap_populate()
    
    In support of providing struct page for large persistent memory
    capacities, use struct vmem_altmap to change the default policy for
    allocating memory for the memmap array.  The default vmemmap_populate()
    allocates page table storage area from the page allocator.  Given
    persistent memory capacities relative to DRAM it may not be feasible to
    store the memmap in 'System Memory'.  Instead vmem_altmap represents
    pre-allocated "device pages" to satisfy vmemmap_alloc_block_buf()
    requests.

In PFN_MODE_PMEM (and only then), we use the altmap (don't see a way to
configure it).


BUT that case is completely different from the "System RAM" mode. The memmap
of an NVDIMM in pmem mode is barely used by core-mm (i.e., not the buddy).

In comparison, if the buddy and everybody else works on the memmap in
"System RAM", it's much more significant if that resides on slow memory.


Looking at

commit 9b6e63cbf85b89b2dbffa4955dbf2df8250e5375
Author: Michal Hocko <mhocko@xxxxxxxx>
Date:   Tue Oct 3 16:16:19 2017 -0700

    mm, page_alloc: add scheduling point to memmap_init_zone
    
    memmap_init_zone gets a pfn range to initialize and it can be really
    large resulting in a soft lockup on non-preemptible kernels
    
      NMI watchdog: BUG: soft lockup - CPU#31 stuck for 23s! [kworker/u642:5:1720]
      [...]
      task: ffff88ecd7e902c0 ti: ffff88eca4e50000 task.ti: ffff88eca4e50000
      RIP: move_pfn_range_to_zone+0x185/0x1d0
      [...]
      Call Trace:
        devm_memremap_pages+0x2c7/0x430
        pmem_attach_disk+0x2fd/0x3f0 [nd_pmem]
        nvdimm_bus_probe+0x64/0x110 [libnvdimm]


It's hard to tell if that was only required due to the memmap for these devices
being that large, or also partially because the access to the memmap is slower
that it makes a real difference.


I recall that we're also often using ZONE_MOVABLE on such slow memory
to not end up placing other kernel data structures on there: especially,
user space page tables as I've been told.


@Dan, any insight on the performance aspects when placing the memmap on
(slow) memory and having that memory be consumed by the buddy where we frequently
operate on the memmap?

--
Cheers,

David / dhildenb