Re: [PATCH v2 1/6] mm/arm64: constify pte_get*() and contpte get logic
From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Date: 2026-08-04 10:55:46
Also in:
linux-arm-kernel, linux-fsdevel, linux-mm, lkml
On Mon, Aug 03, 2026 at 05:43:55PM +0100, Pedro Falcato wrote:
None of the contpte code needs write access to the PTEs.
This seems like a very broad statement? Is that actually true? I see a bunch of ptep's that aren't const-ified, so you should explain why those couldn't be converted. Also you add a new contpte_align_down() macro, you should mention that it the commit message, explain why it was needed. In general more needed here :) it'd be ok if it was a truly trivial change that was all obvious but you're changing some pte_t *'s and not others so it's clearly not.
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Signed-off-by: Pedro Falcato <pfalcato@suse.de> --- arch/arm64/include/asm/pgtable.h | 10 +++++----- arch/arm64/mm/contpte.c | 11 ++++++++--- 2 files changed, 13 insertions(+), 8 deletions(-)diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index a2681d755358..043bc0649cee 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h@@ -378,7 +378,7 @@ static inline void __set_pte(pte_t *ptep, pte_t pte) __set_pte_complete(pte); } -static inline pte_t __ptep_get(pte_t *ptep) +static inline pte_t __ptep_get(const pte_t *ptep) { return READ_ONCE(*ptep); }@@ -1652,8 +1652,8 @@ extern void __contpte_try_fold(struct mm_struct *mm, unsigned long addr, pte_t *ptep, pte_t pte); extern void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr, pte_t *ptep, pte_t pte); -extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte); -extern pte_t contpte_ptep_get_lockless(pte_t *orig_ptep); +extern pte_t contpte_ptep_get(const pte_t *ptep, pte_t orig_pte); +extern pte_t contpte_ptep_get_lockless(const pte_t *orig_ptep);
This is (very) nitty but - not sure on the policy on extern's (Will/Catalin?) - but in mm we drop them when we touch the code since you don't need them these days :)
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extern void contpte_set_ptes(struct mm_struct *mm, unsigned long addr, pte_t *ptep, pte_t pte, unsigned int nr); extern void contpte_clear_full_ptes(struct mm_struct *mm, unsigned long addr,@@ -1732,7 +1732,7 @@ static inline unsigned int pte_batch_hint(pte_t *ptep, pte_t pte) */ #define ptep_get ptep_get -static inline pte_t ptep_get(pte_t *ptep) +static inline pte_t ptep_get(const pte_t *ptep) { pte_t pte = __ptep_get(ptep);@@ -1743,7 +1743,7 @@ static inline pte_t ptep_get(pte_t *ptep) } #define ptep_get_lockless ptep_get_lockless -static inline pte_t ptep_get_lockless(pte_t *ptep) +static inline pte_t ptep_get_lockless(const pte_t *ptep) { pte_t pte = __ptep_get(ptep);diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c index 2de12656b4d8..3a5d6937fb51 100644 --- a/arch/arm64/mm/contpte.c +++ b/arch/arm64/mm/contpte.c@@ -26,6 +26,11 @@ static inline pte_t *contpte_align_down(pte_t *ptep) return PTR_ALIGN_DOWN(ptep, sizeof(*ptep) * CONT_PTES); } +#define contpte_align_down(ptep) \ + _Generic((ptep), \ + const pte_t *: (const pte_t *) contpte_align_down((pte_t *) (ptep)), \ + pte_t *: contpte_align_down((pte_t *) ptep))
I really hate these _Generic() helper things. So ugly. And it's a pretty horrid cast now :( Was it not possible to const-ify further to just be able to constify contpte_align_down itself? It also seems to contradict the claim that contpte doesn't need write-access to pte's since you're going to lengths to allow non-const pte_t * here. You should cover off why this was necessary in the commit message as above. Anyway PTR_ALIGN_DOWN() is already const-safe so couldn't you anyway just collapse this to: #define contpte_align_down(ptep) \ PTR_ALIGN_DOWN(ptep, sizeof(*(ptep)) * CONT_PTES) Then describe in the commit message why you need to handle both cases?
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+ static inline pte_t *contpte_align_addr_ptep(unsigned long *start, unsigned long *end, pte_t *ptep, unsigned int nr)@@ -310,7 +315,7 @@ void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr, } EXPORT_SYMBOL_GPL(__contpte_try_unfold); -pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte) +pte_t contpte_ptep_get(const pte_t *ptep, pte_t orig_pte) { /* * Gather access/dirty bits, which may be populated in any of the ptes@@ -367,7 +372,7 @@ static inline bool contpte_is_consistent(pte_t pte, unsigned long pfn, pgprot_val(prot) == pgprot_val(orig_prot); } -pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) +pte_t contpte_ptep_get_lockless(const pte_t *orig_ptep) { /* * The ptep_get_lockless() API requires us to read and return *orig_ptep@@ -386,10 +391,10 @@ pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) * because it is not part of a contpte range. */ + const pte_t *ptep;
Nit but this is breaking the reverse xmas tree isn't it?
pgprot_t orig_prot; unsigned long pfn; pte_t orig_pte; - pte_t *ptep; pte_t pte; int i; -- 2.55.0
-- Cheers, Lorenzo