[PATCH 09/11] riscv: mm: Clarify ctor mm argument in alloc_{pte,pmd}_late
From: Kevin Brodsky <hidden>
Date: 2025-03-17 14:22:12
Also in:
linux-arch, linux-arm-kernel, linux-m68k, linux-mm, linux-riscv, linux-s390, lkml, sparclinux
Subsystem:
risc-v architecture, the rest · Maintainers:
Paul Walmsley, Palmer Dabbelt, Albert Ou, Linus Torvalds
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 <redacted>
---
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