[PATCH for-mm 1/2 v2] mm/internal: Implement no-op mlock_page_drain() for !CONFIG_MMU

[Hugh Dickins <hughd@google.com>]

From: SeongJae Park <sj@kernel.org>

Commit 4b3b8bd6c8287 ("mm/munlock: mlock_page() munlock_page() batch by
pagevec") in -mm tree[1] implements 'mlock_page_drain()' under
CONFIG_MMU only, but the function is used by 'lru_add_drain_cpu()',
which defined outside of CONFIG_MMU.  As a result, below build error
occurs.

    /linux/mm/swap.c: In function 'lru_add_drain_cpu':
    /linux/mm/swap.c:637:2: error: implicit declaration of function 'mlock_page_drain' [-Werror=implicit-function-declaration]
      637 |  mlock_page_drain(cpu);
          |  ^~~~~~~~~~~~~~~~
    cc1: some warnings being treated as errors
    /linux/scripts/Makefile.build:289: recipe for target 'mm/swap.o' failed

This commit fixes it by implementing no-op 'mlock_page_drain()' for
!CONFIG_MMU case, similar to 'mlock_new_page()'.

[1] https://www.ozlabs.org/~akpm/mmotm/broken-out/mm-munlock-mlock_page-munlock_page-batch-by-pagevec.patch

[hughd: add need_mlock_page_drain() stub too]
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
---
Andrew, Stephen, please add as fix to
mm-munlock-mlock_page-munlock_page-batch-by-pagevec.patch
Thanks!

 mm/internal.h |    2 ++
 1 file changed, 2 insertions(+)

--- a/mm/internal.h
+++ b/mm/internal.h
@@ -508,6 +508,8 @@ static inline void mlock_vma_page(struct page *page,
 static inline void munlock_vma_page(struct page *page,
 			struct vm_area_struct *vma, bool compound) { }
 static inline void mlock_new_page(struct page *page) { }
+static inline bool need_mlock_page_drain(int cpu) { return false; }
+static inline void mlock_page_drain(int cpu) { }
 static inline void vunmap_range_noflush(unsigned long start, unsigned long end)
 {
 }