Re: [PATCH v5 0/5] implement "memmap on memory" feature on s390

[Alexander Gordeev]

On Tue, Nov 28, 2023 at 03:56:34PM -0800, Andrew Morton wrote:

Hi Andrew,

> The cover letter doesn't acutally have a description of what "memmap on
> memory" *is*.  A nice overview to help readers understand what they're
> about to look at.  A description of what value this feature brings to
> our users.  Use-cases.  That sort of thing.
> 
> I guess the [1/N] changelog covers it, but it's hard to tell.  It isn't
> exactly broad-sweep overview.
> 
> Probably something short would suffice.  There are plenty of examples
> on the mailing list, please take a look and send us something?

Sumanth is on vacation, I will try to answer.

This series brings "memmap on memory" support to s390 platform.
That is - allocate 'struct pages' array describing a memory block
being onlined not from available system memory, but from the
memory block itself.

This improves the current situation on s390, where struct pages
for all memory that potentially can be added to the system, is
preallocated, so that memory online will always succeed but at
the cost of significant memory consumption.

Unlike other architectures, the challenge s390 faces is the memory
block being onlined is not accessible at the time of hotplug event.
To make it physically accessible two new MEM_PREPARE_ONLINE and
MEM_FINISH_OFFLINE memory notifiers are added. That allows to succeed
"memmap on memory" initialization.

For the existing architectures the two new notifiers are unknown
and they are not affected in any way.

Thanks!