On Tue 21-01-20 15:08:39, David Rientjes wrote: > On Mon, 20 Jan 2020, Michal Hocko wrote: > > > > > > When migrating memcg charges of thp memory, there are two possibilities: > > > > > > > > > > (1) The underlying compound page is mapped by a pmd and thus does is not > > > > > on a deferred split queue (it's mapped), or > > > > > > > > > > (2) The compound page is not mapped by a pmd and is awaiting split on a > > > > > deferred split queue. > > > > > > > > > > The current charge migration implementation does *not* migrate charges for > > > > > thp memory on the deferred split queue, it only migrates charges for pages > > > > > that are mapped by a pmd. > > > > > > > > > > Thus, to migrate charges, the underlying compound page cannot be on a > > > > > deferred split queue; no list manipulation needs to be done in > > > > > mem_cgroup_move_account(). > > > > > > > > > > With the current code, the underlying compound page is moved to the > > > > > deferred split queue of the memcg its memory is not charged to, so > > > > > susbequent reclaim will consider these pages for the wrong memcg. Remove > > > > > the deferred split queue handling in mem_cgroup_move_account() entirely. > > > > > > > > I believe this still doesn't describe the underlying problem to the full > > > > extent. What happens with the page on the deferred list when it > > > > shouldn't be there in fact? Unless I am missing something deferred_split_scan > > > > will simply split that huge page. Which is a bit unfortunate but nothing > > > > really critical. This should be mentioned in the changelog. > > > > > > > > > > Are you referring to a compound page on the deferred split queue before a > > > task is moved? I'm not sure this is within the scope of Wei's patch.. > > > this is simply preventing a page from being moved to the deferred split > > > queue of a memcg that it is not charged to. Is there a concern about why > > > this code can be removed or a suggestion on something else it should be > > > doing instead? > > > > No, I do not have any concern about the patch itslef. It is that the > > changelog doesn't decribe the user visible effect. All I am saying is > > that the current code splits THPs of moved pages under memory pressure > > even if that is not needed. And that is a clear bug. > > Ah, gotcha. I tried to do this in the final paragraph of my amedment to > Wei's patch and why it's important that this is marked as stable. I considered "susbequent reclaim will consider these pages for the wrong memcg." quite unclear TBH. > The current code in 5.4 from commit 87eaceb3faa59 places any migrated > compound page onto the deferred split queue of the destination memcg > regardless of whether it has a mapping pmd > (list_empty(page_deferred_list()) was already false) or it does not have a > mapping pmd (but is now on the wrong queue). For the latter, > can_split_huge_page() can help for the actual split but not for the > removal of the page that is now erroneously on the queue. Does that mean that those fully mapped THPs are not going to be split? > For the former, > memcg reclaim would not see the pages that it should split under memcg > pressure so we'll see the same memcg oom conditions we saw before the > deferred split shrinker became SHRINKER_MEMCG_AWARE: unnecessary ooms. OK, this is yet another user visibile effect and it would be better to mention it explicitly in the changelog. -- Michal Hocko SUSE Labs
