[PATCH 09/11] riscv: mm: Clarify ctor mm argument in alloc_{pte,pmd}_late

[Kevin Brodsky]

pagetable_{pte,pmd}_ctor(mm, ptdesc) skip the ptlock initialisation
if mm is &init_mm. To avoid unnecessary overhead, it is therefore
preferable to pass the actual mm associated to the PTE/PMD.

Unfortunately, this proves challenging for alloc_{pte,pmd}_late() as
the associated mm is not available at the point where they are
called - in fact not even top-level functions like
create_pgd_mapping() are passed the mm. As a result they both call
the ctor with NULL as mm; this is safe but potentially wasteful.

This is not a new situation, but let's add a couple of comments to
clarify it.

Signed-off-by: Kevin Brodsky <kevin.brodsky at arm.com>
---
 arch/riscv/mm/init.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 703c3648cfa9..fb18940113f2 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -409,6 +409,11 @@ static phys_addr_t __meminit alloc_pte_late(uintptr_t va)
 {
 	struct ptdesc *ptdesc = pagetable_alloc(GFP_KERNEL & ~__GFP_HIGHMEM, 0);
 
+	/*
+	 * We do not know which mm the PTE page is associated to at this point.
+	 * Passing NULL to the ctor is the safe option, though it may result
+	 * in unnecessary work (e.g. initialising the ptlock for init_mm).
+	 */
 	BUG_ON(!ptdesc || !pagetable_pte_ctor(NULL, ptdesc));
 	return __pa((pte_t *)ptdesc_address(ptdesc));
 }
@@ -489,6 +494,7 @@ static phys_addr_t __meminit alloc_pmd_late(uintptr_t va)
 {
 	struct ptdesc *ptdesc = pagetable_alloc(GFP_KERNEL & ~__GFP_HIGHMEM, 0);
 
+	/* See comment in alloc_pte_late() regarding NULL passed the ctor */
 	BUG_ON(!ptdesc || !pagetable_pmd_ctor(NULL, ptdesc));
 	return __pa((pmd_t *)ptdesc_address(ptdesc));
 }
-- 
2.47.0