On Mon 20-01-20 13:10:56, 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. -- Michal Hocko SUSE Labs
