Re: [PATCH] cgroup/dmem: return error when failing to set dmem.max

[Maarten Lankhorst]

Hey,

Den 2026-03-18 kl. 20:34, skrev Thadeu Lima de Souza Cascardo:
> page_counter_set_max may return -EBUSY in case the current usage is above
> the new max. When writing to dmem.max, this error is ignored and the new
> max is not set.
> 
> Return as soon as setting one of the regions max limit fails. This keeps
> with the current behavior of returning when one of the region names is not
> valid.
> 
> After this fix, setting a max value below the current value returns -EBUSY.
> 
>  # cat dmem.current
>  drm/0000:04:00.0/vram 1060864
>  # echo drm/0000:04:00.0/vram 0 > dmem.max
>  -bash: echo: write error: Device or resource busy
>  #
> 
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxx>
The semantics of dmemcg should not substantially differ from the memory cgroup
controller. I believe the memory cgroup controller does allow setting a lower
max, and will evict until below the new max.

See mm/memcontrol.c:memory_max_write

We should probably do the same in dmemcg instead, although we currently have no
mechanism to evict, setting a new lower max at least prevents future allocations
from failing.

I believe we should have a similar loop in dmemcg, and allow ttm to evict until
the new max is reached.

Kind regards,
~Maarten Lankhorst