memcg: mem_cgroup_charge never NULL

My memcgroup patch to fix hang with shmem/tmpfs added NULL page handling to
mem_cgroup_charge_common.  It seemed convenient at the time, but hard to
justify now: there's a perfectly appropriate swappage to charge and uncharge
instead, this is not on any hot path through shmem_getpage, and no performance
hit was observed from the slight extra overhead.

So revert that NULL page handling from mem_cgroup_charge_common; and make it
clearer by bringing page_cgroup_assign_new_page_cgroup into its body - that
was a helper I found more of a hindrance to understanding.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: David Rientjes <rientjes@google.com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Hirokazu Takahashi <taka@valinux.co.jp>
Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 90b576cbc06e..3372bc579e89 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1370,14 +1370,17 @@ repeat:
                         shmem_swp_unmap(entry);
                         spin_unlock(&info->lock);
                         unlock_page(swappage);
-                        page_cache_release(swappage);
                         if (error == -ENOMEM) {
                                 /* allow reclaim from this memory cgroup */
-                                error = mem_cgroup_cache_charge(NULL,
+                                error = mem_cgroup_cache_charge(swappage,
                                         current->mm, gfp & ~__GFP_HIGHMEM);
-                                if (error)
+                                if (error) {
+                                        page_cache_release(swappage);
                                         goto failed;
+                                }
+                                mem_cgroup_uncharge_page(swappage);
                         }
+                        page_cache_release(swappage);
                         goto repeat;
                 }
         } else if (sgp == SGP_READ && !filepage) {