Split page table locks are not used for pgtables associated to
init_mm, at any level. pte_alloc_kernel() does not call
ptlock_init() as a result. There is however no separate alloc/free
functions for kernel PMDs, and pmd_ptlock_init() is called
unconditionally. When ALLOC_SPLIT_PTLOCKS is true (e.g. 32-bit
architectures or if CONFIG_PREEMPT_RT is selected), this results in
unnecessary dynamic memory allocation every time a kernel PMD is
allocated.

Now that pagetable_pmd_ctor() is passed the associated mm, we can
easily remove this overhead by skipping pmd_ptlock_init() if the
pgtable is associated to init_mm. No special-casing is needed on the
dtor path, as ptlock_free() is already called unconditionally for
all levels. (ptlock_free() is a no-op unless a ptlock was allocated
for the given PTP.)

Signed-off-by: Kevin Brodsky <redacted>
---
 include/linux/mm.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ee31ffd7ead2..4759da9cd633 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -3125,7 +3125,7 @@ static inline spinlock_t *pmd_lock(struct mm_struct *mm, pmd_t *pmd)
 static inline bool pagetable_pmd_ctor(struct mm_struct *mm,
 				      struct ptdesc *ptdesc)
 {
-	if (!pmd_ptlock_init(ptdesc))
+	if (mm != &init_mm && !pmd_ptlock_init(ptdesc))
 		return false;
 	ptdesc_pmd_pts_init(ptdesc);
 	__pagetable_ctor(ptdesc);

-- 
2.47.0