Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

[Minchan Kim]

On Fri, Mar 13, 2020 at 09:05:46AM +0100, Michal Hocko wrote:
> On Thu 12-03-20 19:08:51, Minchan Kim wrote:
> > On Thu, Mar 12, 2020 at 09:41:55PM +0100, Michal Hocko wrote:
> > > On Thu 12-03-20 13:16:02, Minchan Kim wrote:
> > > > On Thu, Mar 12, 2020 at 09:22:48AM +0100, Michal Hocko wrote:
> > > [...]
> > > > > From eca97990372679c097a88164ff4b3d7879b0e127 Mon Sep 17 00:00:00 2001
> > > > > From: Michal Hocko <mhocko@xxxxxxxx>
> > > > > Date: Thu, 12 Mar 2020 09:04:35 +0100
> > > > > Subject: [PATCH] mm: do not allow MADV_PAGEOUT for CoW pages
> > > > > 
> > > > > Jann has brought up a very interesting point [1]. While shared pages are
> > > > > excluded from MADV_PAGEOUT normally, CoW pages can be easily reclaimed
> > > > > that way. This can lead to all sorts of hard to debug problems. E.g.
> > > > > performance problems outlined by Daniel [2]. There are runtime
> > > > > environments where there is a substantial memory shared among security
> > > > > domains via CoW memory and a easy to reclaim way of that memory, which
> > > > > MADV_{COLD,PAGEOUT} offers, can lead to either performance degradation
> > > > > in for the parent process which might be more privileged or even open
> > > > > side channel attacks. The feasibility of the later is not really clear
> > > > 
> > > > I am not sure it's a good idea to mention performance stuff because
> > > > it's rather arguble. You and Johannes already pointed it out when I sbumit
> > > > early draft which had shared page filtering out logic due to performance
> > > > reason. You guys suggested the shared pages has higher chance to be touched
> > > > so that if it's really hot pages, that whould keep in the memory. I agree.
> > > 
> > > Yes, the hot memory is likely to be referenced but the point was an
> > > unexpected latency because of the major fault. I have to say that I have
> > 
> > I don't understand your point here. If it's likely to be referenced
> > among several processes, it doesn't have the major fault latency.
> > What's your point here?
> 
> a) the particular CoW page might be cold enough to be reclaimed and b)

If it is, that means it's *cold* so it's really worth to be reclaimed.

> nothing really prevents the MADV_PAGEOUT to be called faster than the
> reference bit being readded.

Yeb, that's undesirable. I should admit it was not intended when I implemented
PAGEOUT. The thing is page_check_references clears access bit of pte for every
process are sharing the page so that two times MADV_PAGEOUT from a process could
evict the page. That's the really bug. It shouldn't have cleared the
access bit for other processes. What I wanted to do with MADV_PAGEOUT is just
check the reference from the other processes without clearing access bit of pte
and if it found other process pte has access bit, just bail out.

Okay so you're right. Current implementation could cause performance impact
since MADV_PAGEOUT unconditionally clear the access bit of other processes so
your patch will fix it.

Thanks.