Re: [PATCH 0/5] memcg/kmem: switch to white list policy

[Vladimir Davydov <vdavydov-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>]

On Mon, Nov 09, 2015 at 03:08:32PM +0100, Michal Hocko wrote:
...
> > Therefore this patch switches to the white list policy. Now kmalloc
> > users have to explicitly opt in by passing __GFP_ACCOUNT flag.
> > 
> > Currently, the list of accounted objects is quite limited and only
> > includes those allocations that (1) are known to be easily triggered
> > from userspace and (2) can fail gracefully (for the full list see patch
> > no. 5) and it still misses many object types. However, accounting only
> > those objects should be a satisfactory approximation of the behavior we
> > used to have for most sane workloads.
> 
> I am _all_ for this semantic I am just not sure what to do with the
> legacy kmem controller. Can we change its semantic? If we cannot do that

I think we can. If somebody reports a "bug" caused by this change, i.e.
basically notices that something that used to be accounted is not any
longer, it will be trivial to fix by adding __GFP_ACCOUNT where
appropriate. If it is not, e.g. if accounting of objects of a particular
type leads to intense false-sharing, we would end up disabling
accounting for it anyway.

> we would have to distinguish legacy and unified hierarchies during
> runtime and add the flag automagically for the first one (that would
> however require to keep __GFP_NOACCOUNT as well) which is all as clear
> as mud. But maybe the workloads which are using kmem legacy API can cope
> with that.
> 
> Anyway if we go this way then I think the kmem accounting would be safe
> to be enabled by default with the cgroup2.
> 
> > Thanks,
> > 
> > Vladimir Davydov (5):
> >   Revert "kernfs: do not account ino_ida allocations to memcg"
> >   Revert "gfp: add __GFP_NOACCOUNT"
> 
> The patch ordering would break the bisectability. I would simply squash

How's that? AFAICS the kernel should compile after any first N=1..5
patches of the series applied.

> both places into the patch which replaces the flag.
> 

IMO it is more readable the way it is, but I don't insist.

Thanks,
Vladimir

> >   memcg: only account kmem allocations marked as __GFP_ACCOUNT
> >   vmalloc: allow to account vmalloc to memcg
> >   Account certain kmem allocations to memcg