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

[Michal Hocko <mhocko@kernel.org>]

On Tue 28-05-19 09:25:13, Konstantin Khlebnikov wrote:
> On 27.05.2019 17:39, Michal Hocko wrote:
> > 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.
> 
> madvise(NULL, size, MADV_STOCKPILE) is the same as memory allocation and
> freeing immediately, but without pinning memory and provoking oom.
>
> So, there is shouldn't be any isolation or security issues.
> 
> At least probably it should be limited with portion of limit (like half)
> instead of whole limit as it does now.

I do not think so. If a process is running inside a memcg then it is
a subject of a limit and that implies an isolation. What you are
proposing here is to allow escaping that restriction unless I am missing
something. Just consider the following setup

		root (total memory = 2G)
		 / \
           (1G) A   B (1G)
                   / \
           (500M) C   D (500M)

all of them used up close to the limit and a process inside D requests
shrinking to 250M. Unless I am misunderstanding this implementation
will shrink D, B root to 250M (which means reclaiming C and A as well)
and then globally if that was not sufficient. So you have allowed D to
"allocate" 1,75G of memory effectively, right?
 
> > 
> > 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?
> 
> For now this is out of my use cease. This could be done in userspace
> with multiple daemons in different contexts and connection between them.
> In this case each daemon should apply pressure only its own level.

Do you expect all daemons to agree on their shrinking target? Could you
elaborate? I simply do not see how this can work with memcgs lower in
the hierarchy having a smaller limit than their parents.
-- 
Michal Hocko
SUSE Labs