[PATCH] mm/memcg: remove useless check on page->mem_cgroup

[Alex Shi <alex.shi@linux.alibaba.com>]

Since readahead page will be charged on memcg too. We don't need to
check this exception now. Rmove them is safe as all user pages are
charged before use.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: cgroups@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
 mm/memcontrol.c | 21 ++++-----------------
 1 file changed, 4 insertions(+), 17 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e84c2b5596f2..9e44ae22d591 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1322,12 +1322,7 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd
 	}
 
 	memcg = page->mem_cgroup;
-	/*
-	 * Swapcache readahead pages are added to the LRU - and
-	 * possibly migrated - before they are charged.
-	 */
-	if (!memcg)
-		memcg = root_mem_cgroup;
+	VM_BUG_ON_PAGE(!memcg, page);
 
 	mz = mem_cgroup_page_nodeinfo(memcg, page);
 	lruvec = &mz->lruvec;
@@ -6897,10 +6892,8 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
 	if (newpage->mem_cgroup)
 		return;
 
-	/* Swapcache readahead pages can get replaced before being charged */
 	memcg = oldpage->mem_cgroup;
-	if (!memcg)
-		return;
+	VM_BUG_ON_PAGE(!memcg, oldpage);
 
 	/* Force-charge the new page. The old one will be freed soon */
 	nr_pages = thp_nr_pages(newpage);
@@ -7094,10 +7087,7 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 		return;
 
 	memcg = page->mem_cgroup;
-
-	/* Readahead page, never charged */
-	if (!memcg)
-		return;
+	VM_BUG_ON_PAGE(!memcg, page);
 
 	/*
 	 * In case the memcg owning these pages has been offlined and doesn't
@@ -7158,10 +7148,7 @@ int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
 		return 0;
 
 	memcg = page->mem_cgroup;
-
-	/* Readahead page, never charged */
-	if (!memcg)
-		return 0;
+	VM_BUG_ON_PAGE(!memcg, page);
 
 	if (!entry.val) {
 		memcg_memory_event(memcg, MEMCG_SWAP_FAIL);
-- 
1.8.3.1