On Fri 24-04-20 12:51:03, Johannes Weiner wrote:
On Fri, Apr 24, 2020 at 06:21:03PM +0200, Michal Hocko wrote:
On Fri 24-04-20 11:10:13, Johannes Weiner wrote:
On Fri, Apr 24, 2020 at 04:29:58PM +0200, Michal Hocko wrote:
On Fri 24-04-20 09:14:50, Johannes Weiner wrote:
On Thu, Apr 23, 2020 at 02:16:29AM -0400, Yafang Shao wrote:
This patch is an improvement of a previous version[1], as the previous version is not easy to understand. This issue persists in the newest kernel, I have to resend the fix. As the implementation is changed, I drop Roman's ack from the previous version.
Now that I understand the problem, I much prefer the previous version.
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 745697906ce3..2bf91ae1e640 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -6332,8 +6332,19 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root, if (!root) root = root_mem_cgroup;
- if (memcg == root)
- if (memcg == root) {
/** The cgroup is the reclaim root in this reclaim* cycle, and therefore not protected. But it may have* stale effective protection values from previous* cycles in which it was not the reclaim root - for* example, global reclaim followed by limit reclaim.* Reset these values for mem_cgroup_protection().*/memcg->memory.emin = 0;memcg->memory.elow = 0;- }
Could you be more specific why you prefer this over the mem_cgroup_protection which doesn't change the effective value? Isn't it easier to simply ignore effective value for the reclaim roots?
Because now both mem_cgroup_protection() and mem_cgroup_protected() have to know about the reclaim root semantics, instead of just the one central place.
Yes this is true but it is also potentially overwriting the state with a parallel reclaim which can lead to surprising results
Checking in mem_cgroup_protection() doesn't avoid the fundamental race:
root `- A (low=2G, elow=2G, max=3G) `- A1 (low=2G, elow=2G)
If A does limit reclaim while global reclaim races, the memcg == root check in mem_cgroup_protection() will reliably calculate the "right" scan value for A, which has no pages, and the wrong scan value for A1 where the memory actually is.
I am sorry but I do not see how A1 would get wrong scan value. - Global reclaim - A.elow = 2G - A1.elow = min(A1.low, A1.usage) ; if (A.children_low_usage < A.elow)
- A reclaim. - A.elow = stale/undefined - A1.elow = A1.low
if mem_cgroup_protection returns 0 for A's reclaim targeting A (assuming the check is there) then not a big deal as there are no pages there as you say.
Let's compare the GR (global reclaim), AR (A reclaim). GR(A1.elow) <= AR(A1.elow) by definition, right? For A1.low overcommitted we have min(A1.low, A1.usage) * A.elow / A.children_low_usage <= min(A1.low, A1.usage) because A.elow <= A.children_low_usage
so in both cases we have GR(A1.elow) <= AR(A1.elow) which means that racing reclaims will behave sanely because the protection for the external pressure pressure is not violated. A is going to reclaim A1 less than the global reclaim but that should be OK.
Or what do I miss?
I'm okay with fixing the case where a really old left-over value is used by target reclaim.
I don't see a point in special casing this one instance of a fundamental race condition at the expense of less robust code.
I am definitely not calling to fragment the code. I do agree that having a special case in mem_cgroup_protection is quite non-intuitive. The existing code is quite hard to reason about in its current form as we can see. If we can fix all that in mem_cgroup_protected then no objections from me at all.