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

[Michal Hocko]

On Tue 28-05-19 10:30:12, Konstantin Khlebnikov wrote:
> On 28.05.2019 9:51, Michal Hocko wrote:
> > 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?
> 
> It shrinks not 'size' memory - only while usage + size > limit.
> So, after reclaiming 250M in D all other levels will have 250M free.

Could you define the exact semantic? Ideally something for the manual
page please?
-- 
Michal Hocko
SUSE Labs