Re: [PATCH RFC] mm/madvise: implement MADV_STOCKPILE (kswapd from user space)

[Michal Hocko <mhocko@kernel.org>]

On Mon 27-05-19 16:21:56, Michal Hocko wrote:
> On Mon 27-05-19 16:12:23, Michal Hocko wrote:
> > [Cc linux-api. Please always cc this list when proposing a new user
> >  visible api. Keeping the rest of the email intact for reference]
> > 
> > On Mon 27-05-19 13:05:58, Konstantin Khlebnikov wrote:
> [...]
> > > This implements manual kswapd-style memory reclaim initiated by userspace.
> > > It reclaims both physical memory and cgroup pages. It works in context of
> > > task who calls syscall madvise thus cpu time is accounted correctly.
> 
> I do not follow. Does this mean that the madvise always reclaims from
> the memcg the process is member of?

OK, I've had a quick look at the implementation (the semantic should be
clear from the patch descrition btw.) and it goes all the way up the
hierarchy and finally try to impose the same limit to the global state.
This doesn't really make much sense to me. For few reasons.

First of all it breaks isolation where one subgroup can influence a
different hierarchy via parent reclaim.

I also have a problem with conflating the global and memcg states. Does
it really make any sense to have the same target to the global state
as per-memcg? How are you supposed to use this interface to shrink a
particular memcg or for the global situation with a proportional
distribution to all memcgs?

There also doens't seem to be anything about security model for this
operation. There is no capability check from a quick look. Is it really
safe to expose such a functionality for a common user?

Last but not least, I am not really convinced that madvise is a proper
interface. It stretches the API which is address range based and it has
per-process implications.
-- 
Michal Hocko
SUSE Labs