Re: Will the recent memory leak fixes be backported to longterm kernels?

[Roman Gushchin <guro@fb.com>]

On Fri, Nov 02, 2018 at 05:51:47PM +0100, Michal Hocko wrote:
> On Fri 02-11-18 16:22:41, Roman Gushchin wrote:
> > On Fri, Nov 02, 2018 at 05:13:14PM +0100, Michal Hocko wrote:
> > > On Fri 02-11-18 15:48:57, Roman Gushchin wrote:
> > > > On Fri, Nov 02, 2018 at 09:03:55AM +0100, Michal Hocko wrote:
> > > > > On Fri 02-11-18 02:45:42, Dexuan Cui wrote:
> > > > > [...]
> > > > > > I totally agree. I'm now just wondering if there is any temporary workaround,
> > > > > > even if that means we have to run the kernel with some features disabled or
> > > > > > with a suboptimal performance?
> > > > > 
> > > > > One way would be to disable kmem accounting (cgroup.memory=nokmem kernel
> > > > > option). That would reduce the memory isolation because quite a lot of
> > > > > memory will not be accounted for but the primary source of in-flight and
> > > > > hard to reclaim memory will be gone.
> > > > 
> > > > In my experience disabling the kmem accounting doesn't really solve the issue
> > > > (without patches), but can lower the rate of the leak.
> > > 
> > > This is unexpected. 90cbc2508827e was introduced to address offline
> > > memcgs to be reclaim even when they are small. But maybe you mean that
> > > we still leak in an absence of the memory pressure. Or what does prevent
> > > memcg from going down?
> > 
> > There are 3 independent issues which are contributing to this leak:
> > 1) Kernel stack accounting weirdness: processes can reuse stack accounted to
> > different cgroups. So basically any running process can take a reference to any
> > cgroup.
> 
> yes, but kmem accounting should rule that out, right? If not then this
> is a clear bug and easy to backport because that would mean to add a
> missing memcg_kmem_enabled check.

Yes, you're right, disabling kmem accounting should mitigate this problem.

> 
> > 2) We do forget to scan the last page in the LRU list. So if we ended up with
> > 1-page long LRU, it can stay there basically forever.
> 
> Why 
> 		/*
> 		 * If the cgroup's already been deleted, make sure to
> 		 * scrape out the remaining cache.
> 		 */
> 		if (!scan && !mem_cgroup_online(memcg))
> 			scan = min(size, SWAP_CLUSTER_MAX);
> 
> in get_scan_count doesn't work for that case?

No, it doesn't. Let's look at the whole picture:

		size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx);
		scan = size >> sc->priority;
		/*
		 * If the cgroup's already been deleted, make sure to
		 * scrape out the remaining cache.
		 */
		if (!scan && !mem_cgroup_online(memcg))
			scan = min(size, SWAP_CLUSTER_MAX);

If size == 1, scan == 0 => scan = min(1, 32) == 1.
And after proportional adjustment we'll have 0.

So, disabling kmem accounting mitigates 2 other issues, but not this one.

Anyway, I'd prefer to wait a bit for test results, and backport the whole
series as a whole.

Thanks!