Re: [RFC 0/3] cgroups: Add support for pinned device memory

[Maarten Lankhorst <dev@lankhorst.se>]

Hey,

Den 2025-08-26 kl. 16:20, skrev Michal Koutný:
> Hello Maarten.
> 
> On Tue, Aug 19, 2025 at 01:49:33PM +0200, Maarten Lankhorst <dev@lankhorst.se> wrote:
>> Implementation details:
>>
>> For each cgroup up until the root cgroup, the 'min' limit is checked
>> against currently effectively pinned value. If the value will go above
>> 'min', the pinning attempt is rejected.
> 
> How is pinning different from setting a 'min' limit (from a user
> perspective)?
It's related, in fact you have to set the 'min' limit first.

The 'pinned' allows you to pick /which/ memory falls under the 'min' limit.

>>
>> Pinned memory is handled slightly different and affects calculating
>> effective min/low values. Pinned memory is subtracted from both,
>> and needs to be added afterwards when calculating.
>>
>> This is because increasing the amount of pinned memory, the amount of
>> free min/low memory decreases for all cgroups that are part of the
>> hierarchy.
> 
> What is supposed to happen with pinned memory after cgroup removal?
I think for accounting purposes pinned memory stays pinned,
otherwise the idea of pinning is lost. However when you kill all
processes in the cgroup, that should solve itself eventually.

> I find the page_counter changes little bit complex without understanding
> of the difference between min and pinned. Should this be conceptually
> similar to memory.stat:unevictable? Or rather mlock(2)? So far neither
> of those needed interaction with min/low values (in memcg).
You could in theory implement mlockall using the 'min' values too.

The page counter changes implement the following:

Lets say you have this tree with 'min' values.

      / '5' A
X'6' -- '5' B
      \ '5' C

Effective min without pinned pages:
      / '2' A
X'6' -- '2' B
      \ '2' C

Now 'B' pins 3 pages:

Effective min:
         / '1' A
X'3+3p' -- '1' B (1 + 3 pinned pages makes effective min 4)
         \ '1' C

Same for applies to effective 'low' calculations.

Kind regards,
~Maarten