On 02/01/2026 16:14, Yeoreum Yun wrote:

Hi Ryan,

quoted

quoted

linear_map_split_to_ptes() currently allocates page tables while
splitting the linear mapping into PTEs under stop_machine() using GFP_ATOMIC.

This is fine for non-PREEMPT_RT configurations.
However, it becomes problematic on PREEMPT_RT, because
generic memory allocation/free APIs (e.g. pgtable_alloc(), __get_free_pages(), etc.)
cannot be called from a non-preemptible context, except for the _nolock() variants.
This is because generic memory allocation/free paths are sleepable,
as they rely on spin_lock(), which becomes sleepable on PREEMPT_RT.

In other words, even calling pgtable_alloc() with GFP_ATOMIC is not permitted
in __linear_map_split_to_pte() when it is executed by the stopper thread,
 where preemption is disabled on PREEMPT_RT.

To address this, the required number of page tables is first collected
and preallocated, and the preallocated page tables are then used
when splitting the linear mapping in __linear_map_split_to_pte().

Fixes: 3df6979d222b ("arm64: mm: split linear mapping if BBML2 unsupported on secondary CPUs")
Signed-off-by: Yeoreum Yun <redacted>

Looks good from my perspective.

I have a couple more small comments below. With those addressed:

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>

quoted

---
 arch/arm64/mm/mmu.c | 204 +++++++++++++++++++++++++++++++++++---------
 1 file changed, 166 insertions(+), 38 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 4b4908ae189b..cc086e91a506 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c

@@ -529,18 +529,14 @@ static void early_create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
 		panic("Failed to create page tables\n");
 }

-static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm, gfp_t gfp,
-				       enum pgtable_type pgtable_type)
-{
-	/* Page is zeroed by init_clear_pgtable() so don't duplicate effort. */
-	struct ptdesc *ptdesc = pagetable_alloc(gfp & ~__GFP_ZERO, 0);
-	phys_addr_t pa;
-
-	if (!ptdesc)
-		return INVALID_PHYS_ADDR;
-
-	pa = page_to_phys(ptdesc_page(ptdesc));
+static struct ptdesc **split_pgtables;
+static unsigned long split_pgtables_count;
+static unsigned long split_pgtables_idx;

I think these could all be __initdata, if you make
pgd_pgtable_get_preallocated() __init (see below) ?

I don't think so since range_split_to_ptes() couldn't be __init.
That's why there is warning while compiling below:

  WARNING: modpost: vmlinux: section mismatch in reference: range_split_to_ptes+0x3c (section: .text) -> pgd_pgtable_get_preallocated (section: .init.text)

Ahh ok, the compiler beats me :)

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quoted

+static __always_inline void __pgd_pgtable_init(struct mm_struct *mm,

Is there a reason for __always_inline? If not, I think it's preferable to just
leave it static and let the compiler decide.

Okay. I'll remove __always_inline. Thanks.

quoted

quoted

+					       struct ptdesc *ptdesc,
+					       enum pgtable_type pgtable_type)
+{
 	switch (pgtable_type) {
 	case TABLE_PTE:
 		BUG_ON(!pagetable_pte_ctor(mm, ptdesc));

@@ -555,26 +551,49 @@ static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm, gfp_t gfp,
 		pagetable_p4d_ctor(ptdesc);
 		break;
 	}
-
-	return pa;
 }

-static phys_addr_t
-pgd_pgtable_alloc_init_mm_gfp(enum pgtable_type pgtable_type, gfp_t gfp)
+static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm,
+				       enum pgtable_type pgtable_type)
 {
-	return __pgd_pgtable_alloc(&init_mm, gfp, pgtable_type);
+	/* Page is zeroed by init_clear_pgtable() so don't duplicate effort. */
+	struct ptdesc *ptdesc = pagetable_alloc(GFP_PGTABLE_KERNEL & ~__GFP_ZERO, 0);
+
+	if (!ptdesc)
+		return INVALID_PHYS_ADDR;
+
+	__pgd_pgtable_init(mm, ptdesc, pgtable_type);
+
+	return page_to_phys(ptdesc_page(ptdesc));
 }

-static phys_addr_t __maybe_unused
+static phys_addr_t
 pgd_pgtable_alloc_init_mm(enum pgtable_type pgtable_type)
 {
-	return pgd_pgtable_alloc_init_mm_gfp(pgtable_type, GFP_PGTABLE_KERNEL);
+	return __pgd_pgtable_alloc(&init_mm, pgtable_type);
 }

 static phys_addr_t
 pgd_pgtable_alloc_special_mm(enum pgtable_type pgtable_type)
 {
-	return  __pgd_pgtable_alloc(NULL, GFP_PGTABLE_KERNEL, pgtable_type);
+	return  __pgd_pgtable_alloc(NULL, pgtable_type);
+}
+
+static phys_addr_t
+pgd_pgtable_get_preallocated(enum pgtable_type pgtable_type)

I think this could probably be __init?

See above.

quoted

quoted

+{
+	struct ptdesc *ptdesc;
+
+	if (WARN_ON(split_pgtables_idx >= split_pgtables_count))
+		return INVALID_PHYS_ADDR;
+
+	ptdesc = split_pgtables[split_pgtables_idx++];
+	if (!ptdesc)
+		return INVALID_PHYS_ADDR;
+
+	__pgd_pgtable_init(&init_mm, ptdesc, pgtable_type);
+
+	return page_to_phys(ptdesc_page(ptdesc));
 }

 static void split_contpte(pte_t *ptep)

@@ -586,7 +605,9 @@ static void split_contpte(pte_t *ptep)
 		__set_pte(ptep, pte_mknoncont(__ptep_get(ptep)));
 }

-static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
+static int split_pmd(pmd_t *pmdp, pmd_t pmd,
+		     pgtable_alloc_t pgtable_alloc,
+		     bool to_cont)

nit: this will easily fit in 2 lines and still be within 80 chars:

Okay. I'll change it.

quoted

static int split_pmd(pmd_t *pmdp, pmd_t pmd, pgtable_alloc_t pgtable_alloc,
		     bool to_cont)

quoted

 {
 	pmdval_t tableprot = PMD_TYPE_TABLE | PMD_TABLE_UXN | PMD_TABLE_AF;
 	unsigned long pfn = pmd_pfn(pmd);

@@ -595,7 +616,7 @@ static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
 	pte_t *ptep;
 	int i;

-	pte_phys = pgd_pgtable_alloc_init_mm_gfp(TABLE_PTE, gfp);
+	pte_phys = pgtable_alloc(TABLE_PTE);
 	if (pte_phys == INVALID_PHYS_ADDR)
 		return -ENOMEM;
 	ptep = (pte_t *)phys_to_virt(pte_phys);

@@ -630,7 +651,9 @@ static void split_contpmd(pmd_t *pmdp)
 		set_pmd(pmdp, pmd_mknoncont(pmdp_get(pmdp)));
 }

-static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp, bool to_cont)
+static int split_pud(pud_t *pudp, pud_t pud,
+		     pgtable_alloc_t pgtable_alloc,
+		     bool to_cont)

nit: same comment.

Thanks for your review :D
BTW, except the __init related comments, Could I add R-b tag
after fixing others?

Yes, please add my R-b.

--
Sincerely,
Yeoreum Yun