Re: [PATCH v1 3/4] memblock: add MEMBLOCK_DRIVER_MANAGED to mimic IORESOURCE_SYSRAM_DRIVER_MANAGED

[Mike Rapoport <rppt@kernel.org>]

On Fri, Oct 01, 2021 at 10:04:24AM +0200, David Hildenbrand wrote:
> On 30.09.21 23:21, Mike Rapoport wrote:
> > On Wed, Sep 29, 2021 at 06:54:01PM +0200, David Hildenbrand wrote:
> > > On 29.09.21 18:39, Mike Rapoport wrote:
> > > > Hi,
> > > > 
> > > > On Mon, Sep 27, 2021 at 05:05:17PM +0200, David Hildenbrand wrote:
> > > > > Let's add a flag that corresponds to IORESOURCE_SYSRAM_DRIVER_MANAGED.
> > > > > Similar to MEMBLOCK_HOTPLUG, most infrastructure has to treat such memory
> > > > > like ordinary MEMBLOCK_NONE memory -- for example, when selecting memory
> > > > > regions to add to the vmcore for dumping in the crashkernel via
> > > > > for_each_mem_range().
> > > > Can you please elaborate on the difference in semantics of MEMBLOCK_HOTPLUG
> > > > and MEMBLOCK_DRIVER_MANAGED?
> > > > Unless I'm missing something they both mark memory that can be unplugged
> > > > anytime and so it should not be used in certain cases. Why is there a need
> > > > for a new flag?
> > > 
> > > In the cover letter I have "Alternative B: Reuse MEMBLOCK_HOTPLUG.
> > > MEMBLOCK_HOTPLUG serves a different purpose, though.", but looking into the
> > > details it won't work as is.
> > > 
> > > MEMBLOCK_HOTPLUG is used to mark memory early during boot that can later get
> > > hotunplugged again and should be placed into ZONE_MOVABLE if the
> > > "movable_node" kernel parameter is set.
> > > 
> > > The confusing part is that we talk about "hotpluggable" but really mean
> > > "hotunpluggable": the reason is that HW flags DIMM slots that can later be
> > > hotplugged as "hotpluggable" even though there is already something
> > > hotplugged.
> > 
> > MEMBLOCK_HOTPLUG name is indeed somewhat confusing, but still it's core
> > meaning "this memory may be removed" which does not differ from what
> > IORESOURCE_SYSRAM_DRIVER_MANAGED means.
> > 
> > MEMBLOCK_HOTPLUG regions are indeed placed into ZONE_MOVABLE, but more
> > importantly, they are avoided when we allocate memory from memblock.
> > 
> > So, in my view, both flags mean that the memory may be removed and it
> > should not be used for certain types of allocations.
> 
> The semantics are different:
> 
> MEMBLOCK_HOTPLUG: memory is indicated as "System RAM" in the
> firmware-provided memory map and added to the system early during boot; we
> want this memory to be managed by ZONE_MOVABLE with "movable_node" set on
> the kernel command line, because only then we want it to be hotpluggable
> again. kexec *has to* indicate this memory to the second kernel and can
> place kexec-images on this memory. After memory hotunplug, kexec has to be
> re-armed.
> 
> MEMBLOCK_DRIVER_MANAGED: memory is not indicated as System RAM" in the
> firmware-provided memory map; this memory is always detected and added to
> the system by a driver; memory might not actually be physically
> hotunpluggable and the ZONE selection does not depend on "movable_core".
> kexec *must not* indicate this memory to the second kernel and *must not*
> place kexec-images on this memory.

Ok, this clarifies.
This explanation should be a part of the changelog. The sentences about the
zone selection could be probably skipped, because they are less important
for this case. E.g something like:

MEMBLOCK_HOTPLUG: memory is indicated as "System RAM" in the
firmware-provided memory map and added to the system early during boot;
kexec *has to* indicate this memory to the second kernel and can place
kexec-images on this memory. After memory hotunplug, kexec has to be
re-armed.

MEMBLOCK_DRIVER_MANAGED: memory is not indicated as "System RAM" in the
firmware-provided memory map; this memory is always detected and added to
the system by a driver; memory might not actually be physically
hotunpluggable.  kexec *must not* indicate this memory to the second kernel
and *must not* place kexec-images on this memory.

-- 
Sincerely yours,
Mike.