memcg: do not replicate get_mem_cgroup_from_mm in __mem_cgroup_try_charge

__mem_cgroup_try_charge duplicates get_mem_cgroup_from_mm for charges
which came without a memcg.  The only reason seems to be a tiny
optimization when css_tryget is not called if the charge can be consumed
from the stock.  Nevertheless css_tryget is very cheap since it has been
reworked to use per-cpu counting so this optimization doesn't give us
anything these days.

So let's drop the code duplication so that the code is more readable.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 87c3ec3..7480022 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2697,52 +2697,14 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 again:
        if (*ptr) { /* css should be a valid one */
                memcg = *ptr;
-               if (mem_cgroup_is_root(memcg))
-                       goto done;
-               if (consume_stock(memcg, nr_pages))
-                       goto done;
                css_get(&memcg->css);
        } else {
-               struct task_struct *p;
-
-               rcu_read_lock();
-               p = rcu_dereference(mm->owner);
-               /*
-                * Because we don't have task_lock(), "p" can exit.
-                * In that case, "memcg" can point to root or p can be NULL with
-                * race with swapoff. Then, we have small risk of mis-accouning.
-                * But such kind of mis-account by race always happens because
-                * we don't have cgroup_mutex(). It's overkill and we allo that
-                * small race, here.
-                * (*) swapoff at el will charge against mm-struct not against
-                * task-struct. So, mm->owner can be NULL.
-                */
-               memcg = mem_cgroup_from_task(p);
-               if (!memcg)
-                       memcg = root_mem_cgroup;
-               if (mem_cgroup_is_root(memcg)) {
-                       rcu_read_unlock();
-                       goto done;
-               }
-               if (consume_stock(memcg, nr_pages)) {
-                       /*
-                        * It seems dagerous to access memcg without css_get().
-                        * But considering how consume_stok works, it's not
-                        * necessary. If consume_stock success, some charges
-                        * from this memcg are cached on this cpu. So, we
-                        * don't need to call css_get()/css_tryget() before
-                        * calling consume_stock().
-                        */
-                       rcu_read_unlock();
-                       goto done;
-               }
-               /* after here, we may be blocked. we need to get refcnt */
-               if (!css_tryget(&memcg->css)) {
-                       rcu_read_unlock();
-                       goto again;
-               }
-               rcu_read_unlock();
+               memcg = get_mem_cgroup_from_mm(mm);
        }
+       if (mem_cgroup_is_root(memcg))
+               goto done;
+       if (consume_stock(memcg, nr_pages))
+               goto done;
 
        do {
                bool invoke_oom = oom && !nr_oom_retries;
@@ -2778,8 +2740,8 @@ again:
 
        if (batch > nr_pages)
                refill_stock(memcg, batch - nr_pages);
-       css_put(&memcg->css);
 done:
+       css_put(&memcg->css);
        *ptr = memcg;
        return 0;
 nomem: