Re: [PATCH 1/2] mm, memcg: Avoid stale protection values when cgroup is above protection

[Yafang Shao]

On Wed, Apr 29, 2020 at 10:03 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Wed, Apr 29, 2020 at 12:15:10PM +0200, Michal Hocko wrote:
> > On Tue 28-04-20 19:26:47, Chris Down wrote:
> > > From: Yafang Shao <laoar.shao@xxxxxxxxx>
> > >
> > > A cgroup can have both memory protection and a memory limit to isolate
> > > it from its siblings in both directions - for example, to prevent it
> > > from being shrunk below 2G under high pressure from outside, but also
> > > from growing beyond 4G under low pressure.
> > >
> > > Commit 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim")
> > > implemented proportional scan pressure so that multiple siblings in
> > > excess of their protection settings don't get reclaimed equally but
> > > instead in accordance to their unprotected portion.
> > >
> > > During limit reclaim, this proportionality shouldn't apply of course:
> > > there is no competition, all pressure is from within the cgroup and
> > > should be applied as such. Reclaim should operate at full efficiency.
> > >
> > > However, mem_cgroup_protected() never expected anybody to look at the
> > > effective protection values when it indicated that the cgroup is above
> > > its protection. As a result, a query during limit reclaim may return
> > > stale protection values that were calculated by a previous reclaim cycle
> > > in which the cgroup did have siblings.
> > >
> > > When this happens, reclaim is unnecessarily hesitant and potentially
> > > slow to meet the desired limit. In theory this could lead to premature
> > > OOM kills, although it's not obvious this has occurred in practice.
> >
> > Thanks this describes the underlying problem. I would be also explicit
> > that the issue should be visible only on tail memcgs which have both
> > max/high and protection configured and the effect depends on the
> > difference between the two (the smaller it is the largrger the effect).
> >
> > There is no mention about the fix. The patch resets effective values for
> > the reclaim root and I've had some concerns about that
> > http://lkml.kernel.org/r/20200424162103.GK11591@xxxxxxxxxxxxxxx
> > Johannes has argued that other races are possible and I didn't get to
> > think about it thoroughly. But this patch is introducing a new
> > possibility of breaking protection. If we want to have a quick and
> > simple fix that would be easier to backport to older kernels then I
> > would feel much better if we simply workedaround the problem as
> > suggested earlier http://lkml.kernel.org/r/20200423061629.24185-1-laoar.shao@xxxxxxxxx
> > We can rework the effective values calculation to be more robust against
> > races on top of that because this is likely a more tricky thing to do.
>
> Well, can you please *do* think more thoroughly about what I wrote,
> instead of pushing for an alternative patch on gut feeling alone?
>
> Especially when you imply that this should be a stable patch.
>
> Not only does your alternative patch not protect against the race you
> are worried about, the race itself doesn't matter. Racing reclaimers
> will write their competing views of the world into the shared state on
> all other levels anyway.
>
> And that's okay. If the configuration and memory usage is such that
> there is at least one reclaimer that scans without any protection
> (like a limit reclaimer), it's not a problem when a second reclaimer
> that meant to do protected global reclaim will also do one iteration
> without protection. It's no different than if a second thread had
> entered limit reclaim through another internal allocation.
>
> There is no semantical violation with the race in your patch or the
> race in this patch. Any effective protection that becomes visible is
> 1) permitted by the configuration, but 2) also triggered *right now*
> by an acute need to reclaim memory with these parameters.
>
> The *right now* part is important. That's what's broken before either
> patch, and that's what we're fixing: to see really, really *old* stale
> that might not be representative of the config semantics anymore.
>
> Since you haven't linked to my email, here is my counter argument to
> the alternative patch "fixing" this race somehow.
>
> A reclaim:
>
>   root
>      `- A (low=2G, max=3G -> elow=0)
>         `- A1 (low=0G -> elow=0)
>
> Global reclaim:
>
>   root
>      `- A (low=2G, max=3G -> elow=2G)
>         `- A1 (low=0G -> elow=2G)
>
> During global reclaim, A1 is supposed to have 2G effective low
> protection. If A limit reclaim races, it can set A1's elow to
> 0.

Before the commit  8a931f801340c2be ("mm: memcontrol: recursive
memory.low protection"), the A1's elow should be 0, while after this
commit A1's elow is 2G.
That is a behavior change.

Then this case gives us another example why accessing emin and elow in
the very deap reclaiming code (get_scan_count) is the root of ALL
EVIL.

>  Global reclaim will now query mem_cgroup_protection(root, A1), the
> root == memcg check you insist we add will fail and it'll reclaim A1
> without protection.
>
> The alternative patch is nothing except slightly worse code.



-- 
Thanks
Yafang