Message-Id: <20250613134352.65994-2-dev.jain@arm.com>
Date: Fri, 13 Jun 2025 19:13:51 +0530
From: Dev Jain <dev.jain@....com>
To: akpm@...ux-foundation.org,
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Cc: lorenzo.stoakes@...cle.com,
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	Dev Jain <dev.jain@....com>
Subject: [PATCH v3 1/2] arm64: pageattr: Use pagewalk API to change memory permissions

arm64 currently changes permissions on vmalloc objects locklessly, via
apply_to_page_range, whose limitation is to deny changing permissions for
block mappings. Therefore, we move away to use the generic pagewalk API,
thus paving the path for enabling huge mappings by default on kernel space
mappings, thus leading to more efficient TLB usage. However, the API
currently enforces the init_mm.mmap_lock to be held. To avoid the
unnecessary bottleneck of the mmap_lock for our usecase, this patch
extends this generic API to be used locklessly, so as to retain the
existing behaviour for changing permissions. Apart from this reason, it is
noted at [1] that KFENCE can manipulate kernel pgtable entries during
softirqs. It does this by calling set_memory_valid() -> __change_memory_common().
This being a non-sleepable context, we cannot take the init_mm mmap lock.

Add comments to highlight the conditions under which we can use the
lockless variant - no underlying VMA, and the user having exclusive control
over the range, thus guaranteeing no concurrent access.

Since arm64 cannot handle kernel live mapping splitting without BBML2,
we require that the start and end of a given range lie on block mapping
boundaries. Return -EINVAL in case a partial block mapping is detected;
add a corresponding comment in ___change_memory_common() to warn that
eliminating such a condition is the responsibility of the caller.

apply_to_page_range() currently performs all pte level callbacks while in
lazy mmu mode. Since arm64 can optimize performance by batching barriers
when modifying kernel pgtables in lazy mmu mode, we would like to continue
to benefit from this optimisation. Unfortunately walk_kernel_page_table_range()
does not use lazy mmu mode. However, since the pagewalk framework is not
allocating any memory, we can safely bracket the whole operation inside
lazy mmu mode ourselves. Therefore, wrap the call to
walk_kernel_page_table_range() with the lazy MMU helpers.

[1] https://lore.kernel.org/linux-arm-kernel/89d0ad18-4772-4d8f-ae8a-7c48d26a927e@arm.com/

Signed-off-by: Dev Jain <dev.jain@....com>
---
 arch/arm64/mm/pageattr.c | 157 +++++++++++++++++++++++++++++++--------
 include/linux/pagewalk.h |   3 +
 mm/pagewalk.c            |  26 +++++++
 3 files changed, 154 insertions(+), 32 deletions(-)

diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 04d4a8f676db..cfc5279f27a2 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -8,6 +8,7 @@
 #include <linux/mem_encrypt.h>
 #include <linux/sched.h>
 #include <linux/vmalloc.h>
+#include <linux/pagewalk.h>
 
 #include <asm/cacheflush.h>
 #include <asm/pgtable-prot.h>
@@ -20,6 +21,99 @@ struct page_change_data {
 	pgprot_t clear_mask;
 };
 
+static ptdesc_t set_pageattr_masks(ptdesc_t val, struct mm_walk *walk)
+{
+	struct page_change_data *masks = walk->private;
+
+	val &= ~(pgprot_val(masks->clear_mask));
+	val |= (pgprot_val(masks->set_mask));
+
+	return val;
+}
+
+static int pageattr_pgd_entry(pgd_t *pgd, unsigned long addr,
+			      unsigned long next, struct mm_walk *walk)
+{
+	pgd_t val = pgdp_get(pgd);
+
+	if (pgd_leaf(val)) {
+		if (WARN_ON_ONCE((next - addr) != PGDIR_SIZE))
+			return -EINVAL;
+		val = __pgd(set_pageattr_masks(pgd_val(val), walk));
+		set_pgd(pgd, val);
+		walk->action = ACTION_CONTINUE;
+	}
+
+	return 0;
+}
+
+static int pageattr_p4d_entry(p4d_t *p4d, unsigned long addr,
+			      unsigned long next, struct mm_walk *walk)
+{
+	p4d_t val = p4dp_get(p4d);
+
+	if (p4d_leaf(val)) {
+		if (WARN_ON_ONCE((next - addr) != P4D_SIZE))
+			return -EINVAL;
+		val = __p4d(set_pageattr_masks(p4d_val(val), walk));
+		set_p4d(p4d, val);
+		walk->action = ACTION_CONTINUE;
+	}
+
+	return 0;
+}
+
+static int pageattr_pud_entry(pud_t *pud, unsigned long addr,
+			      unsigned long next, struct mm_walk *walk)
+{
+	pud_t val = pudp_get(pud);
+
+	if (pud_leaf(val)) {
+		if (WARN_ON_ONCE((next - addr) != PUD_SIZE))
+			return -EINVAL;
+		val = __pud(set_pageattr_masks(pud_val(val), walk));
+		set_pud(pud, val);
+		walk->action = ACTION_CONTINUE;
+	}
+
+	return 0;
+}
+
+static int pageattr_pmd_entry(pmd_t *pmd, unsigned long addr,
+			      unsigned long next, struct mm_walk *walk)
+{
+	pmd_t val = pmdp_get(pmd);
+
+	if (pmd_leaf(val)) {
+		if (WARN_ON_ONCE((next - addr) != PMD_SIZE))
+			return -EINVAL;
+		val = __pmd(set_pageattr_masks(pmd_val(val), walk));
+		set_pmd(pmd, val);
+		walk->action = ACTION_CONTINUE;
+	}
+
+	return 0;
+}
+
+static int pageattr_pte_entry(pte_t *pte, unsigned long addr,
+			      unsigned long next, struct mm_walk *walk)
+{
+	pte_t val = __ptep_get(pte);
+
+	val = __pte(set_pageattr_masks(pte_val(val), walk));
+	__set_pte(pte, val);
+
+	return 0;
+}
+
+static const struct mm_walk_ops pageattr_ops = {
+	.pgd_entry	= pageattr_pgd_entry,
+	.p4d_entry	= pageattr_p4d_entry,
+	.pud_entry	= pageattr_pud_entry,
+	.pmd_entry	= pageattr_pmd_entry,
+	.pte_entry	= pageattr_pte_entry,
+};
+
 bool rodata_full __ro_after_init = IS_ENABLED(CONFIG_RODATA_FULL_DEFAULT_ENABLED);
 
 bool can_set_direct_map(void)
@@ -37,22 +131,7 @@ bool can_set_direct_map(void)
 		arm64_kfence_can_set_direct_map() || is_realm_world();
 }
 
-static int change_page_range(pte_t *ptep, unsigned long addr, void *data)
-{
-	struct page_change_data *cdata = data;
-	pte_t pte = __ptep_get(ptep);
-
-	pte = clear_pte_bit(pte, cdata->clear_mask);
-	pte = set_pte_bit(pte, cdata->set_mask);
-
-	__set_pte(ptep, pte);
-	return 0;
-}
-
-/*
- * This function assumes that the range is mapped with PAGE_SIZE pages.
- */
-static int __change_memory_common(unsigned long start, unsigned long size,
+static int ___change_memory_common(unsigned long start, unsigned long size,
 				pgprot_t set_mask, pgprot_t clear_mask)
 {
 	struct page_change_data data;
@@ -61,9 +140,28 @@ static int __change_memory_common(unsigned long start, unsigned long size,
 	data.set_mask = set_mask;
 	data.clear_mask = clear_mask;
 
-	ret = apply_to_page_range(&init_mm, start, size, change_page_range,
-					&data);
+	arch_enter_lazy_mmu_mode();
+
+	/*
+	 * The caller must ensure that the range we are operating on does not
+	 * partially overlap a block mapping. Any such case should either not
+	 * exist, or must be eliminated by splitting the mapping - which for
+	 * kernel mappings can be done only on BBML2 systems.
+	 *
+	 */
+	ret = walk_kernel_page_table_range_lockless(start, start + size,
+						    &pageattr_ops, NULL, &data);
+	arch_leave_lazy_mmu_mode();
+
+	return ret;
+}
 
+static int __change_memory_common(unsigned long start, unsigned long size,
+				  pgprot_t set_mask, pgprot_t clear_mask)
+{
+	int ret;
+
+	ret = ___change_memory_common(start, size, set_mask, clear_mask);
 	/*
 	 * If the memory is being made valid without changing any other bits
 	 * then a TLBI isn't required as a non-valid entry cannot be cached in
@@ -71,6 +169,7 @@ static int __change_memory_common(unsigned long start, unsigned long size,
 	 */
 	if (pgprot_val(set_mask) != PTE_VALID || pgprot_val(clear_mask))
 		flush_tlb_kernel_range(start, start + size);
+
 	return ret;
 }
 
@@ -174,32 +273,26 @@ int set_memory_valid(unsigned long addr, int numpages, int enable)
 
 int set_direct_map_invalid_noflush(struct page *page)
 {
-	struct page_change_data data = {
-		.set_mask = __pgprot(0),
-		.clear_mask = __pgprot(PTE_VALID),
-	};
+	pgprot_t clear_mask = __pgprot(PTE_VALID);
+	pgprot_t set_mask = __pgprot(0);
 
 	if (!can_set_direct_map())
 		return 0;
 
-	return apply_to_page_range(&init_mm,
-				   (unsigned long)page_address(page),
-				   PAGE_SIZE, change_page_range, &data);
+	return ___change_memory_common((unsigned long)page_address(page),
+					PAGE_SIZE, set_mask, clear_mask);
 }
 
 int set_direct_map_default_noflush(struct page *page)
 {
-	struct page_change_data data = {
-		.set_mask = __pgprot(PTE_VALID | PTE_WRITE),
-		.clear_mask = __pgprot(PTE_RDONLY),
-	};
+	pgprot_t set_mask = __pgprot(PTE_VALID | PTE_WRITE);
+	pgprot_t clear_mask = __pgprot(PTE_RDONLY);
 
 	if (!can_set_direct_map())
 		return 0;
 
-	return apply_to_page_range(&init_mm,
-				   (unsigned long)page_address(page),
-				   PAGE_SIZE, change_page_range, &data);
+	return ___change_memory_common((unsigned long)page_address(page),
+					PAGE_SIZE, set_mask, clear_mask);
 }
 
 static int __set_memory_enc_dec(unsigned long addr,
diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index 8ac2f6d6d2a3..79ab8c754dff 100644
--- a/include/linux/pagewalk.h
+++ b/include/linux/pagewalk.h
@@ -132,6 +132,9 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
 int walk_kernel_page_table_range(unsigned long start,
 		unsigned long end, const struct mm_walk_ops *ops,
 		pgd_t *pgd, void *private);
+int walk_kernel_page_table_range_lockless(unsigned long start,
+		unsigned long end, const struct mm_walk_ops *ops,
+		pgd_t *pgd, void *private);
 int walk_page_range_vma(struct vm_area_struct *vma, unsigned long start,
 			unsigned long end, const struct mm_walk_ops *ops,
 			void *private);
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index ff5299eca687..7446984b2154 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -632,6 +632,32 @@ int walk_kernel_page_table_range(unsigned long start, unsigned long end,
 	return walk_pgd_range(start, end, &walk);
 }
 
+/*
+ * Use this function to walk the kernel page tables locklessly. It should be
+ * guaranteed that the caller has exclusive access over the range they are
+ * operating on - that there should be no concurrent access, for example,
+ * changing permissions for vmalloc objects.
+ */
+int walk_kernel_page_table_range_lockless(unsigned long start, unsigned long end,
+		const struct mm_walk_ops *ops, pgd_t *pgd, void *private)
+{
+	struct mm_struct *mm = &init_mm;
+	struct mm_walk walk = {
+		.ops		= ops,
+		.mm		= mm,
+		.pgd		= pgd,
+		.private	= private,
+		.no_vma		= true
+	};
+
+	if (start >= end)
+		return -EINVAL;
+	if (!check_ops_valid(ops))
+		return -EINVAL;
+
+	return walk_pgd_range(start, end, &walk);
+}
+
 /**
  * walk_page_range_debug - walk a range of pagetables not backed by a vma
  * @mm:		mm_struct representing the target process of page table walk
-- 
2.30.2

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