mm: memcg: push !mm handling out to page cache charge function

Only page cache charges can happen without an mm context, so push this
special case out of the inner core and into the cache charge function.

An ancient comment explains that the mm can also be NULL in case the
task is currently being migrated, but that is not actually true with the
current case, so just remove it.

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

mm/memcontrol.c

@@ -2703,15 +2703,6 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 
 	if (gfp_mask & __GFP_NOFAIL)
 		oom = false;
-
-	/*
-	 * We always charge the cgroup the mm_struct belongs to.
-	 * The mm_struct's mem_cgroup changes on task migration if the
-	 * thread group leader migrates. It's possible that mm is not
-	 * set, if so charge the root memcg (happens for pagecache usage).
-	 */
-	if (!*ptr && !mm)
-		*ptr = root_mem_cgroup;
 again:
 	if (*ptr) { /* css should be a valid one */
 		memcg = *ptr;
@@ -4038,6 +4029,12 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
 		return 0;
 
 	if (!PageSwapCache(page)) {
+		/*
+		 * Page cache insertions can happen without an actual
+		 * task context, e.g. during disk probing on boot.
+		 */
+		if (!mm)
+			memcg = root_mem_cgroup;
 		ret = __mem_cgroup_try_charge(mm, gfp_mask, 1, &memcg, true);
 		if (ret != -ENOMEM)
 			__mem_cgroup_commit_charge(memcg, page, 1, type, false);