memcg: fix oops on NULL lru list

While testing force_empty, during an exit_mmap, __mem_cgroup_remove_list
called from mem_cgroup_uncharge_page oopsed on a NULL pointer in the lru list.
 I couldn't see what racing tasks on other cpus were doing, but surmise that
another must have been in mem_cgroup_charge_common on the same page, between
its unlock_page_cgroup and spin_lock_irqsave near done (thanks to that kzalloc
which I'd almost changed to a kmalloc).

Normally such a race cannot happen, the ref_cnt prevents it, the final
uncharge cannot race with the initial charge.  But force_empty buggers the
ref_cnt, that's what it's all about; and thereafter forced pages are
vulnerable to races such as this (just think of a shared page also mapped into
an mm of another mem_cgroup than that just emptied).  And remain vulnerable
until they're freed indefinitely later.

This patch just fixes the oops by moving the unlock_page_cgroups down below
adding to and removing from the list (only possible given the previous patch);
and while we're at it, we might as well make it an invariant that
page->page_cgroup is always set while pc is on lru.

But this behaviour of force_empty seems highly unsatisfactory to me: why have
a ref_cnt if we always have to cope with it being violated (as in the earlier
page migration patch).  We may prefer force_empty to move pages to an orphan
mem_cgroup (could be the root, but better not), from which other cgroups could
recover them; we might need to reverse the locking again; but no time now for
such concerns.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 8 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f72067094e05..8b9f6cae938e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -623,13 +623,13 @@ retry:
                 goto retry;
         }
         page_assign_page_cgroup(page, pc);
-        unlock_page_cgroup(page);
 
         mz = page_cgroup_zoneinfo(pc);
         spin_lock_irqsave(&mz->lru_lock, flags);
         __mem_cgroup_add_list(pc);
         spin_unlock_irqrestore(&mz->lru_lock, flags);
 
+        unlock_page_cgroup(page);
 done:
         return 0;
 out:
@@ -677,14 +677,14 @@ void mem_cgroup_uncharge_page(struct page *page)
         VM_BUG_ON(pc->ref_cnt <= 0);
 
         if (--(pc->ref_cnt) == 0) {
-                page_assign_page_cgroup(page, NULL);
-                unlock_page_cgroup(page);
-
                 mz = page_cgroup_zoneinfo(pc);
                 spin_lock_irqsave(&mz->lru_lock, flags);
                 __mem_cgroup_remove_list(pc);
                 spin_unlock_irqrestore(&mz->lru_lock, flags);
 
+                page_assign_page_cgroup(page, NULL);
+                unlock_page_cgroup(page);
+
                 mem = pc->mem_cgroup;
                 res_counter_uncharge(&mem->res, PAGE_SIZE);
                 css_put(&mem->css);
@@ -736,23 +736,24 @@ void mem_cgroup_page_migration(struct page *page, struct page *newpage)
                 return;
         }
 
-        page_assign_page_cgroup(page, NULL);
-        unlock_page_cgroup(page);
-
         mz = page_cgroup_zoneinfo(pc);
         spin_lock_irqsave(&mz->lru_lock, flags);
         __mem_cgroup_remove_list(pc);
         spin_unlock_irqrestore(&mz->lru_lock, flags);
 
+        page_assign_page_cgroup(page, NULL);
+        unlock_page_cgroup(page);
+
         pc->page = newpage;
         lock_page_cgroup(newpage);
         page_assign_page_cgroup(newpage, pc);
-        unlock_page_cgroup(newpage);
 
         mz = page_cgroup_zoneinfo(pc);
         spin_lock_irqsave(&mz->lru_lock, flags);
         __mem_cgroup_add_list(pc);
         spin_unlock_irqrestore(&mz->lru_lock, flags);
+
+        unlock_page_cgroup(newpage);
 }
 
 /*