[PATCH 2/2] mm/memcg: warning on !memcg after readahead page charged

[Alex Shi]

Add VM_WARN_ON_ONCE_PAGE() macro.

Since readahead page is charged on memcg too, in theory we don't have to
check this exception now. Before safely remove them all, add a warning
for the unexpected !memcg.

Signed-off-by: Alex Shi <alex....@linux.alibaba.com>
Acked-by: Michal Hocko <mho...@suse.com>
Acked-by: Hugh Dickins <hu...@google.com>
Acked-by: Johannes Weiner <han...@cmpxchg.org>
Cc: Johannes Weiner <han...@cmpxchg.org>
Cc: Michal Hocko <mho...@kernel.org>
Cc: Vladimir Davydov <vdavydov....@gmail.com>
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: cgro...@vger.kernel.org
Cc: linux...@kvack.org
Cc: linux-kernel@vger.kernel.org
---
 include/linux/mmdebug.h | 13 +++++++++++++
 mm/memcontrol.c         | 11 ++++-------
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
index 2ad72d2c8cc5..5d0767cb424a 100644
--- a/include/linux/mmdebug.h
+++ b/include/linux/mmdebug.h
@@ -37,6 +37,18 @@
                        BUG();                                          \
                }                                                       \
        } while (0)
+#define VM_WARN_ON_ONCE_PAGE(cond, page)       ({                      \
+       static bool __section(".data.once") __warned;                   \
+       int __ret_warn_once = !!(cond);                                 \
+                                                                       \
+       if (unlikely(__ret_warn_once && !__warned)) {                   \
+               dump_page(page, "VM_WARN_ON_ONCE_PAGE(" __stringify(cond)")");\
+               __warned = true;                                        \
+               WARN_ON(1);                                             \
+       }                                                               \
+       unlikely(__ret_warn_once);                                      \
+})
+
 #define VM_WARN_ON(cond) (void)WARN_ON(cond)
 #define VM_WARN_ON_ONCE(cond) (void)WARN_ON_ONCE(cond)
 #define VM_WARN_ONCE(cond, format...) (void)WARN_ONCE(cond, format)
@@ -48,6 +60,7 @@
 #define VM_BUG_ON_MM(cond, mm) VM_BUG_ON(cond)
 #define VM_WARN_ON(cond) BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN_ON_ONCE(cond) BUILD_BUG_ON_INVALID(cond)
+#define VM_WARN_ON_ONCE_PAGE(cond, page)  BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN_ONCE(cond, format...) BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN(cond, format...) BUILD_BUG_ON_INVALID(cond)
 #endif
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ec13fe4ed407..e46b9f9501c2 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1350,10 +1350,7 @@ 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.
-        */
+       VM_WARN_ON_ONCE_PAGE(!memcg, page);
        if (!memcg)
                memcg = root_mem_cgroup;
 
@@ -6979,8 +6976,8 @@ 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;
+       VM_WARN_ON_ONCE_PAGE(!memcg, oldpage);
        if (!memcg)
                return;
 
@@ -7180,7 +7177,7 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t 
entry)
 
        memcg = page->mem_cgroup;
 
-       /* Readahead page, never charged */
+       VM_WARN_ON_ONCE_PAGE(!memcg, page);
        if (!memcg)
                return;
 
@@ -7247,7 +7244,7 @@ int mem_cgroup_try_charge_swap(struct page *page, 
swp_entry_t entry)
 
        memcg = page->mem_cgroup;
 
-       /* Readahead page, never charged */
+       VM_WARN_ON_ONCE_PAGE(!memcg, page);
        if (!memcg)
                return 0;
 
-- 
1.8.3.1