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?
