Re: [PATCH 03/11] cgroup: add memory_cma_accounting mount option

On Fri, Aug 21, 2026 at 02:56:55PM -0400, Eric Chanudet wrote:
> Add a cgroup v2 mount option that will be used later in this series to
> enable CMA memory accounting under memcg.
> 
> The option is made opt-in as it will charge allocations to the cgroup's
> overall memory usage, which changes existing expectations.
> 
> Signed-off-by: Eric Chanudet <[email protected]>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 20 ++++++++++++++++++++
>  include/linux/cgroup-defs.h             |  5 +++++
>  kernel/cgroup/cgroup.c                  | 15 ++++++++++++++-
>  3 files changed, 39 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index aed195a71cbf..3dfc13f31a0c 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -246,6 +246,26 @@ cgroup v2 currently supports the following mount options.
>            will not be tracked by the memory controller (even if cgroup
>            v2 is remounted later on).
>  
> +  memory_cma_accounting
> +        Count CMA memory usage towards the cgroup's overall memory usage for
> +        the memory controller, and expose per-CMA-area usage counters. This
> +        behavior is opt-in, so as to not cause regression on existing setups.
> +
> +        A few caveats to keep in mind:
> +
> +        * CMA areas are reserved at boot time, memory is only charged to a
> +          cgroup when a CMA allocation is performed on its behalf.
> +        * CMA pages allocated while this option is not enabled will not be
> +          tracked by the memory controller, even if cgroup v2 is remounted
> +          later on with this option. Charges happen at allocation.

And what would happen if it's the other way around, ie mounted with this
option enabled, then remounted with the option disabled?

Maxime

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