On Wed, Apr 21, 2021 at 03:00:50PM +0800, Muchun Song wrote: > The diagram below shows how to make the page lruvec lock safe when the > LRU pages reparented. > > lock_page_lruvec(page) > retry: > lruvec = mem_cgroup_page_lruvec(page); > > // The page is reparented at this time. > spin_lock(&lruvec->lru_lock); > > if (unlikely(lruvec_memcg(lruvec) != page_memcg(page))) > // Acquired the wrong lruvec lock and need to retry. > // Because this page is on the parent memcg lruvec list. > goto retry; > > // If we reach here, it means that page_memcg(page) is stable. > > memcg_reparent_objcgs(memcg) > // lruvec belongs to memcg and lruvec_parent belongs to parent memcg. > spin_lock(&lruvec->lru_lock); > spin_lock(&lruvec_parent->lru_lock); > > // Move all the pages from the lruvec list to the parent lruvec list. > > spin_unlock(&lruvec_parent->lru_lock); > spin_unlock(&lruvec->lru_lock); > > After we acquire the lruvec lock, we need to check whether the page is > reparented. If so, we need to reacquire the new lruvec lock. On the > routine of the LRU pages reparenting, we will also acquire the lruvec > lock (Will be implemented in the later patch). So page_memcg() cannot > be changed when we hold the lruvec lock. > > Since lruvec_memcg(lruvec) is always equal to page_memcg(page) after > we hold the lruvec lock, lruvec_memcg_debug() check is pointless. So > remove it. > > This is a preparation for reparenting the LRU pages. > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx> Acked-by: Roman Gushchin <guro@xxxxxx> Thanks!
