[PATCH v7 1/3] memcontrol: define root_mem_cgroup for CONFIG_MEMCG=n cases

[Qu Wenruo <wqu@suse.com>]

There is an incoming btrfs patchset, which will use @root_mem_cgroup as
the active cgroup to attach metadata folios to its internal btree
inode, so that btrfs can skip the possibly costly charge for the
internal inode which is only accessible by btrfs itself.

However @root_mem_cgroup is not always defined (not defined for
CONFIG_MEMCG=n case), thus all such callers need to do the extra
handling for different CONFIG_MEMCG settings.

So here we add a special macro definition of root_mem_cgroup, making it
to always be NULL.

The advantage of this, other than pulling the pointer definition out,
is that we will avoid wasting global data section space for such
pointer.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 include/linux/memcontrol.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 030d34e9d117..ae5c78719454 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -346,6 +346,12 @@ enum page_memcg_data_flags {
 
 #define __FIRST_OBJEXT_FLAG	(1UL << 0)
 
+/*
+ * For CONFIG_MEMCG=n case, still define a root_mem_cgroup, but that will
+ * always be NULL and not taking any global data section space.
+ */
+#define root_mem_cgroup		(NULL)
+
 #endif /* CONFIG_MEMCG */
 
 enum objext_flags {
-- 
2.45.2