[PATCH 2/6] mm: Lock kernel page tables before entering lazy MMU mode

[Alexander Gordeev]

As a follow-up to commit 691ee97e1a9d ("mm: fix lazy mmu docs and
usage") take a step forward and protect with a lock not only user,
but also kernel mappings before entering the lazy MMU mode. With
that the semantics of arch_enter|leave_lazy_mmu_mode() callbacks
is consolidated, which allows further simplifications.

The effect of this consolidation is not fully preemptible (Real-Time)
kernels can not enter the context switch while the lazy MMU mode is
active - which is easier to comprehend.

Signed-off-by: Alexander Gordeev <agordeev at linux.ibm.com>
---
 include/linux/pgtable.h | 12 ++++++------
 mm/kasan/shadow.c       |  5 -----
 mm/memory.c             |  5 ++++-
 mm/vmalloc.c            |  6 ++++++
 4 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 0b6e1f781d86..33bf2b13c219 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -224,12 +224,12 @@ static inline int pmd_dirty(pmd_t pmd)
  * a raw PTE pointer after it has been modified are not guaranteed to be
  * up to date.
  *
- * In the general case, no lock is guaranteed to be held between entry and exit
- * of the lazy mode. So the implementation must assume preemption may be enabled
- * and cpu migration is possible; it must take steps to be robust against this.
- * (In practice, for user PTE updates, the appropriate page table lock(s) are
- * held, but for kernel PTE updates, no lock is held). Nesting is not permitted
- * and the mode cannot be used in interrupt context.
+ * For PREEMPT_RT kernels implementation must assume that preemption may
+ * be enabled and cpu migration is possible between entry and exit of the
+ * lazy MMU mode; it must take steps to be robust against this. There is
+ * no such assumption for non-PREEMPT_RT kernels, since both kernel and
+ * user page tables are protected with a spinlock while in lazy MMU mode.
+ * Nesting is not permitted and the mode cannot be used in interrupt context.
  */
 #ifndef __HAVE_ARCH_ENTER_LAZY_MMU_MODE
 #define arch_enter_lazy_mmu_mode()	do {} while (0)
diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index d2c70cd2afb1..45115bd770a9 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -313,12 +313,10 @@ static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
 	__memset(page_to_virt(page), KASAN_VMALLOC_INVALID, PAGE_SIZE);
 	pte = pfn_pte(page_to_pfn(page), PAGE_KERNEL);
 
-	spin_lock(&init_mm.page_table_lock);
 	if (likely(pte_none(ptep_get(ptep)))) {
 		set_pte_at(&init_mm, addr, ptep, pte);
 		data->pages[index] = NULL;
 	}
-	spin_unlock(&init_mm.page_table_lock);
 
 	return 0;
 }
@@ -465,13 +463,10 @@ static int kasan_depopulate_vmalloc_pte(pte_t *ptep, unsigned long addr,
 
 	page = (unsigned long)__va(pte_pfn(ptep_get(ptep)) << PAGE_SHIFT);
 
-	spin_lock(&init_mm.page_table_lock);
-
 	if (likely(!pte_none(ptep_get(ptep)))) {
 		pte_clear(&init_mm, addr, ptep);
 		free_page(page);
 	}
-	spin_unlock(&init_mm.page_table_lock);
 
 	return 0;
 }
diff --git a/mm/memory.c b/mm/memory.c
index 71b3d3f98999..1ddc532b1f13 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3017,6 +3017,7 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
 			pte = pte_offset_kernel(pmd, addr);
 		if (!pte)
 			return err;
+		spin_lock(&init_mm.page_table_lock);
 	} else {
 		if (create)
 			pte = pte_alloc_map_lock(mm, pmd, addr, &ptl);
@@ -3042,7 +3043,9 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
 
 	arch_leave_lazy_mmu_mode();
 
-	if (mm != &init_mm)
+	if (mm == &init_mm)
+		spin_unlock(&init_mm.page_table_lock);
+	else
 		pte_unmap_unlock(mapped_pte, ptl);
 
 	*mask |= PGTBL_PTE_MODIFIED;
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index ab986dd09b6a..57b11000ae36 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -105,6 +105,7 @@ static int vmap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	if (!pte)
 		return -ENOMEM;
 
+	spin_lock(&init_mm.page_table_lock);
 	arch_enter_lazy_mmu_mode();
 
 	do {
@@ -132,6 +133,7 @@ static int vmap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	} while (pte += PFN_DOWN(size), addr += size, addr != end);
 
 	arch_leave_lazy_mmu_mode();
+	spin_unlock(&init_mm.page_table_lock);
 	*mask |= PGTBL_PTE_MODIFIED;
 	return 0;
 }
@@ -359,6 +361,7 @@ static void vunmap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	unsigned long size = PAGE_SIZE;
 
 	pte = pte_offset_kernel(pmd, addr);
+	spin_lock(&init_mm.page_table_lock);
 	arch_enter_lazy_mmu_mode();
 
 	do {
@@ -379,6 +382,7 @@ static void vunmap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	} while (pte += (size >> PAGE_SHIFT), addr += size, addr != end);
 
 	arch_leave_lazy_mmu_mode();
+	spin_unlock(&init_mm.page_table_lock);
 	*mask |= PGTBL_PTE_MODIFIED;
 }
 
@@ -525,6 +529,7 @@ static int vmap_pages_pte_range(pmd_t *pmd, unsigned long addr,
 	if (!pte)
 		return -ENOMEM;
 
+	spin_lock(&init_mm.page_table_lock);
 	arch_enter_lazy_mmu_mode();
 
 	do {
@@ -542,6 +547,7 @@ static int vmap_pages_pte_range(pmd_t *pmd, unsigned long addr,
 	} while (pte++, addr += PAGE_SIZE, addr != end);
 
 	arch_leave_lazy_mmu_mode();
+	spin_unlock(&init_mm.page_table_lock);
 	*mask |= PGTBL_PTE_MODIFIED;
 	return 0;
 }
-- 
2.48.1