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

Michal Hocko
2023-06-15T12:16:56Z

On Thu 15-06-23 19:58:37, 贺中坤 wrote:
> Hi michal, glad to hear from you.
>
> > I am not really deeply familiar with zram implementation nor usage but
> > how is the above allocation going to be charged without __GFP_ACCOUNT in
> > the gfp mask?
>
> Yes，zs_malloc() did not charge compressed memory, even if we add this gfp.
> so we need to implement this function in this patchset. But this flag should be
> used to enable this feature.

Let me check I understand. This patch on its own doesn't really do
anything. You need the zs_malloc support implemented in patch 3 for this
to have any effect. Even with that in place the zs_malloc doesn't follow
the __GFP_ACCOUNT scheme we use for allocation tracking. Correct?

> > Also what exactly is going to happen for the swap backed by the zram
> > device? Your memcg might be hitting the hard limit and therefore
> > swapping out. Wouldn't zs_malloc fail very likely under that condition
> > making the swap effectively unusable?
>
> This is the key point, as i said above, zs_malloc() did not charge
> compressed memory,
> so zs_malloc will not fail under that condition. if the zram swap is
> large enough, zs_malloc
> never fails unless system OOM. so memory.max will be invalidated.

I do not think this is answering my question. Or maybe I just
misunderstand. Let me try again. Say you have a memcg under hard limit
pressure so any further charge is going to fail. How can you reasonably
implement zram back swapout if the memory is charged?

--
Michal Hocko
SUSE Labs