Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

[Minchan Kim]

On Wed, Mar 11, 2020 at 04:53:17PM -0700, Shakeel Butt wrote:
> On Wed, Mar 11, 2020 at 1:45 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >
> > On Tue 10-03-20 15:48:31, Dave Hansen wrote:
> > > Maybe instead of just punting on MADV_PAGEOUT for map_count>1 we should
> > > only let it affect the *local* process.  We could still put the page in
> > > the swap cache, we just wouldn't go do the rmap walk.
> >
> > Is it really worth medling with the reclaim code and special case
> > MADV_PAGEOUT there? I mean it is quite reasonable to have an initial
> > implementation that doesn't really touch shared pages because that can
> > lead to all sorts of hard to debug and unexpected problems. So I would
> > much rather go with a simple patch to check map count first and see
> > whether somebody actually cares about those shared pages and go from
> > there.
> >
> > Minchan, do you want to take my diff and turn it into the proper patch
> > or should I do it.
> >
> 
> What about the remote_madvise(MADV_PAGEOUT)? Will your patch disable
> the pageout from that code path as well for pages with mapcount > 1?

Maybe, not because process_madvise syscall needs more previliedge(ie,
PTRACE_MODE_ATTACH_FSCREDS) so I guess it would be more secure.
So in that case, I want to rely on the LRU chance for shared pages.

With that, the manager process could give a hint to several processes
and finally makes them paging out.

What do you think?