mm: skip ptlock_init() for kernel PMDs

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.)

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Kevin Brodsky <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Andreas Larsson <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Linus Waleij <[email protected]>
Cc: Madhavan Srinivasan <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Qi Zheng <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions