Re: [RFC PATCH 1/3] zram: charge the compressed RAM to the page's memcgroup

[David Hildenbrand]

On 15.06.23 05:48, Zhongkun He wrote:
> The compressed RAM is currently charged to kernel, not to
> any memory cgroup, which is not satisfy our usage scenario.
> if the memory of a task is limited by memcgroup, it will
> swap out the memory to zram swap device when the memory
> is insufficient. In that case, the memory limit will have
> no effect.
>
> So, it should makes sense to charge the compressed RAM to
> the page's memory cgroup.

Interesting. When looking at possible ways to achieve that in a clean 
way, my understanding was that the page might not always be accounted to 
a memcg (e.g., simply some buffer?). Besides the swap->zram case I was 
thinking about other fs->zram case, or just a straight read/write to the 
zram device.

The easiest way to see where that goes wrong I think is 
zram_bvec_write_partial(), where we simply allocate a temporary page.

Either I am missing something important, or this only works in some 
special cases.

I came to the conclusion that the only reasonable way is to assign a 
complete zram device to a single cgroup and have all memory accounted to 
that cgroup. Does also not solve all use cases (especially the 
swap->zram case that might be better offjust using zswap?) but at least 
the memory gets accounted in a more predictable way.

Happy to learn more.

--
Cheers,

David / dhildenb