On Mon 09-01-23 21:38:04, T.J. Mercier wrote:
When a buffer is exported to userspace, use memcg to attribute the buffer to the allocating cgroup until all buffer references are released.
Unlike the dmabuf sysfs stats implementation, this memcg accounting avoids contention over the kernfs_rwsem incurred when creating or removing nodes.
I am not familiar with dmabuf infrastructure so please bear with me. AFAIU this patch adds a dmabuf specific counter to find out the amount of dmabuf memory used. But I do not see any actual charging implemented for that memory.
I have looked at two random users of dma_buf_export cma_heap_allocate and it allocates pages to back the dmabuf (AFAIU) by cma_alloc which doesn't account to memcg, system_heap_allocate uses alloc_largest_available which relies on order_flags which doesn't seem to ever use __GFP_ACCOUNT.
This would mean that the counter doesn't represent any actual memory reflected in the overall memory consumption of a memcg. I believe this is rather unexpected and confusing behavior. While some counters overlap and their sum would exceed the charged memory we do not have any that doesn't correspond to any memory (at least not for non-root memcgs).