Subject: [PATCH v4 6/7] mm: introduce __vm_flags_mod and use it in untrack_pfn

From: Suren Baghdasaryan <[email-masked]>
Date: Thu, 26 Jan 2023 11:37:51 -0800

There are scenarios when vm_flags can be modified without exclusive
mmap_lock, such as:
- after VMA was isolated and mmap_lock was downgraded or dropped
- in exit_mmap when there are no other mm users and locking is unnecessary
Introduce __vm_flags_mod to avoid assertions when the caller takes
responsibility for the required locking.
Pass a hint to untrack_pfn to conditionally use __vm_flags_mod for
flags modification to avoid assertion.

Signed-off-by: Suren Baghdasaryan <[email-masked]>
Acked-by: Michal Hocko <[email-masked]>
---
 arch/x86/mm/pat/memtype.c | 10 +++++++---
 include/linux/mm.h        | 14 ++++++++++++--
 include/linux/pgtable.h   |  5 +++--
 mm/memory.c               | 13 +++++++------
 mm/memremap.c             |  4 ++--
 mm/mmap.c                 | 16 ++++++++++------
 6 files changed, 41 insertions(+), 21 deletions(-)

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index 6ca51b1aa5d9..691bf8934b6f 100644
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -1046,7 +1046,7 @@ void track_pfn_insert(struct vm_area_struct *vma, pgprot_t *prot, pfn_t pfn)
  * can be for the entire vma (in which case pfn, size are zero).
  */
 void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
-		 unsigned long size)
+		 unsigned long size, bool mm_wr_locked)
 {
 	resource_size_t paddr;
 	unsigned long prot;
@@ -1065,8 +1065,12 @@ void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
 		size = vma->vm_end - vma->vm_start;
 	}
 	free_pfn_range(paddr, size);
-	if (vma)
-		vm_flags_clear(vma, VM_PAT);
+	if (vma) {
+		if (mm_wr_locked)
+			vm_flags_clear(vma, VM_PAT);
+		else
+			__vm_flags_mod(vma, 0, VM_PAT);
+	}
 }
 
 /*
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 3c7fc3ecaece..a00fdeb4492d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -656,6 +656,16 @@ static inline void vm_flags_clear(struct vm_area_struct *vma,
 	ACCESS_PRIVATE(vma, __vm_flags) &= ~flags;
 }
 
+/*
+ * Use only if VMA is not part of the VMA tree or has no other users and
+ * therefore needs no locking.
+ */
+static inline void __vm_flags_mod(struct vm_area_struct *vma,
+				  vm_flags_t set, vm_flags_t clear)
+{
+	vm_flags_init(vma, (vma->vm_flags | set) & ~clear);
+}
+
 /*
  * Use only when the order of set/clear operations is unimportant, otherwise
  * use vm_flags_{set|clear} explicitly.
@@ -664,7 +674,7 @@ static inline void vm_flags_mod(struct vm_area_struct *vma,
 				vm_flags_t set, vm_flags_t clear)
 {
 	mmap_assert_write_locked(vma->vm_mm);
-	vm_flags_init(vma, (vma->vm_flags | set) & ~clear);
+	__vm_flags_mod(vma, set, clear);
 }
 
 static inline void vma_set_anonymous(struct vm_area_struct *vma)
@@ -2102,7 +2112,7 @@ static inline void zap_vma_pages(struct vm_area_struct *vma)
 }
 void unmap_vmas(struct mmu_gather *tlb, struct maple_tree *mt,
 		struct vm_area_struct *start_vma, unsigned long start,
-		unsigned long end);
+		unsigned long end, bool mm_wr_locked);
 
 struct mmu_notifier_range;
 
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 5fd45454c073..c63cd44777ec 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1185,7 +1185,8 @@ static inline int track_pfn_copy(struct vm_area_struct *vma)
  * can be for the entire vma (in which case pfn, size are zero).
  */
 static inline void untrack_pfn(struct vm_area_struct *vma,
-			       unsigned long pfn, unsigned long size)
+			       unsigned long pfn, unsigned long size,
+			       bool mm_wr_locked)
 {
 }
 
@@ -1203,7 +1204,7 @@ extern void track_pfn_insert(struct vm_area_struct *vma, pgprot_t *prot,
 			     pfn_t pfn);
 extern int track_pfn_copy(struct vm_area_struct *vma);
 extern void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
-			unsigned long size);
+			unsigned long size, bool mm_wr_locked);
 extern void untrack_pfn_moved(struct vm_area_struct *vma);
 #endif
 
diff --git a/mm/memory.c b/mm/memory.c
index a6316cda0e87..7a04a1130ec1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1613,7 +1613,7 @@ void unmap_page_range(struct mmu_gather *tlb,
 static void unmap_single_vma(struct mmu_gather *tlb,
 		struct vm_area_struct *vma, unsigned long start_addr,
 		unsigned long end_addr,
-		struct zap_details *details)
+		struct zap_details *details, bool mm_wr_locked)
 {
 	unsigned long start = max(vma->vm_start, start_addr);
 	unsigned long end;
@@ -1628,7 +1628,7 @@ static void unmap_single_vma(struct mmu_gather *tlb,
 		uprobe_munmap(vma, start, end);
 
 	if (unlikely(vma->vm_flags & VM_PFNMAP))
-		untrack_pfn(vma, 0, 0);
+		untrack_pfn(vma, 0, 0, mm_wr_locked);
 
 	if (start != end) {
 		if (unlikely(is_vm_hugetlb_page(vma))) {
@@ -1675,7 +1675,7 @@ static void unmap_single_vma(struct mmu_gather *tlb,
  */
 void unmap_vmas(struct mmu_gather *tlb, struct maple_tree *mt,
 		struct vm_area_struct *vma, unsigned long start_addr,
-		unsigned long end_addr)
+		unsigned long end_addr, bool mm_wr_locked)
 {
 	struct mmu_notifier_range range;
 	struct zap_details details = {
@@ -1689,7 +1689,8 @@ void unmap_vmas(struct mmu_gather *tlb, struct maple_tree *mt,
 				start_addr, end_addr);
 	mmu_notifier_invalidate_range_start(&range);
 	do {
-		unmap_single_vma(tlb, vma, start_addr, end_addr, &details);
+		unmap_single_vma(tlb, vma, start_addr, end_addr, &details,
+				 mm_wr_locked);
 	} while ((vma = mas_find(&mas, end_addr - 1)) != NULL);
 	mmu_notifier_invalidate_range_end(&range);
 }
@@ -1723,7 +1724,7 @@ void zap_page_range_single(struct vm_area_struct *vma, unsigned long address,
 	 * unmap 'address-end' not 'range.start-range.end' as range
 	 * could have been expanded for hugetlb pmd sharing.
 	 */
-	unmap_single_vma(&tlb, vma, address, end, details);
+	unmap_single_vma(&tlb, vma, address, end, details, false);
 	mmu_notifier_invalidate_range_end(&range);
 	tlb_finish_mmu(&tlb);
 }
@@ -2492,7 +2493,7 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
 
 	err = remap_pfn_range_notrack(vma, addr, pfn, size, prot);
 	if (err)
-		untrack_pfn(vma, pfn, PAGE_ALIGN(size));
+		untrack_pfn(vma, pfn, PAGE_ALIGN(size), true);
 	return err;
 }
 EXPORT_SYMBOL(remap_pfn_range);
diff --git a/mm/memremap.c b/mm/memremap.c
index 08cbf54fe037..2f88f43d4a01 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -129,7 +129,7 @@ static void pageunmap_range(struct dev_pagemap *pgmap, int range_id)
 	}
 	mem_hotplug_done();
 
-	untrack_pfn(NULL, PHYS_PFN(range->start), range_len(range));
+	untrack_pfn(NULL, PHYS_PFN(range->start), range_len(range), true);
 	pgmap_array_delete(range);
 }
 
@@ -276,7 +276,7 @@ static int pagemap_range(struct dev_pagemap *pgmap, struct mhp_params *params,
 	if (!is_private)
 		kasan_remove_zero_shadow(__va(range->start), range_len(range));
 err_kasan:
-	untrack_pfn(NULL, PHYS_PFN(range->start), range_len(range));
+	untrack_pfn(NULL, PHYS_PFN(range->start), range_len(range), true);
 err_pfn_remap:
 	pgmap_array_delete(range);
 	return error;
diff --git a/mm/mmap.c b/mm/mmap.c
index 3d9b14d5f933..429e42c8fccc 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -78,7 +78,7 @@ core_param(ignore_rlimit_data, ignore_rlimit_data, bool, 0644);
 static void unmap_region(struct mm_struct *mm, struct maple_tree *mt,
 		struct vm_area_struct *vma, struct vm_area_struct *prev,
 		struct vm_area_struct *next, unsigned long start,
-		unsigned long end);
+		unsigned long end, bool mm_wr_locked);
 
 static pgprot_t vm_pgprot_modify(pgprot_t oldprot, unsigned long vm_flags)
 {
@@ -2136,14 +2136,14 @@ static inline void remove_mt(struct mm_struct *mm, struct ma_state *mas)
 static void unmap_region(struct mm_struct *mm, struct maple_tree *mt,
 		struct vm_area_struct *vma, struct vm_area_struct *prev,
 		struct vm_area_struct *next,
-		unsigned long start, unsigned long end)
+		unsigned long start, unsigned long end, bool mm_wr_locked)
 {
 	struct mmu_gather tlb;
 
 	lru_add_drain();
 	tlb_gather_mmu(&tlb, mm);
 	update_hiwater_rss(mm);
-	unmap_vmas(&tlb, mt, vma, start, end);
+	unmap_vmas(&tlb, mt, vma, start, end, mm_wr_locked);
 	free_pgtables(&tlb, mt, vma, prev ? prev->vm_end : FIRST_USER_ADDRESS,
 				 next ? next->vm_start : USER_PGTABLES_CEILING);
 	tlb_finish_mmu(&tlb);
@@ -2391,7 +2391,11 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 			mmap_write_downgrade(mm);
 	}
 
-	unmap_region(mm, &mt_detach, vma, prev, next, start, end);
+	/*
+	 * We can free page tables without write-locking mmap_lock because VMAs
+	 * were isolated before we downgraded mmap_lock.
+	 */
+	unmap_region(mm, &mt_detach, vma, prev, next, start, end, !downgrade);
 	/* Statistics and freeing VMAs */
 	mas_set(&mas_detach, start);
 	remove_mt(mm, &mas_detach);
@@ -2704,7 +2708,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 
 		/* Undo any partial mapping done by a device driver. */
 		unmap_region(mm, &mm->mm_mt, vma, prev, next, vma->vm_start,
-			     vma->vm_end);
+			     vma->vm_end, true);
 	}
 	if (file && (vm_flags & VM_SHARED))
 		mapping_unmap_writable(file->f_mapping);
@@ -3031,7 +3035,7 @@ void exit_mmap(struct mm_struct *mm)
 	tlb_gather_mmu_fullmm(&tlb, mm);
 	/* update_hiwater_rss(mm) here? but nobody should be looking */
 	/* Use ULONG_MAX here to ensure all VMAs in the mm are unmapped */
-	unmap_vmas(&tlb, &mm->mm_mt, vma, 0, ULONG_MAX);
+	unmap_vmas(&tlb, &mm->mm_mt, vma, 0, ULONG_MAX, false);
 	mmap_read_unlock(mm);
 
 	/*
-- 
2.39.1