[PATCH v4 04/20] mm: VMA sequence count

[Laurent Dufour]

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Wrap the VMA modifications (vma_adjust/unmap_page_range) with sequence
counts such that we can easily test if a VMA is changed.

The unmap_page_range() one allows us to make assumptions about
page-tables; when we find the seqcount hasn't changed we can assume
page-tables are still valid.

The flip side is that we cannot distinguish between a vma_adjust() and
the unmap_page_range() -- where with the former we could have
re-checked the vma bounds against the address.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

[Port to 4.12 kernel]
[Build depends on __HAVE_ARCH_CALL_SPF]
[Introduce vm_write_* inline function depending on __HAVE_ARCH_CALL_SPF]
[Fix lock dependency between mapping->i_mmap_rwsem and vma->vm_sequence by
 using vm_raw_write* functions]
Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>

Fix locked by raw function

undo lockdep fix as raw services are now used
---
 include/linux/mm.h       | 41 +++++++++++++++++++++++++++++++++++++++++
 include/linux/mm_types.h |  3 +++
 mm/memory.c              |  2 ++
 mm/mmap.c                | 40 +++++++++++++++++++++++++++++++++++++---
 4 files changed, 83 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index fb3477f7f742..aa06332fc01e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1349,6 +1349,47 @@ static inline int fixup_user_fault(struct task_struct *tsk,
 }
 #endif
 
+#ifdef __HAVE_ARCH_CALL_SPF
+static inline void vm_write_begin(struct vm_area_struct *vma)
+{
+	write_seqcount_begin(&vma->vm_sequence);
+}
+static inline void vm_write_begin_nested(struct vm_area_struct *vma,
+					 int subclass)
+{
+	write_seqcount_begin_nested(&vma->vm_sequence, subclass);
+}
+static inline void vm_write_end(struct vm_area_struct *vma)
+{
+	write_seqcount_end(&vma->vm_sequence);
+}
+static inline void vm_raw_write_begin(struct vm_area_struct *vma)
+{
+	raw_write_seqcount_begin(&vma->vm_sequence);
+}
+static inline void vm_raw_write_end(struct vm_area_struct *vma)
+{
+	raw_write_seqcount_end(&vma->vm_sequence);
+}
+#else
+static inline void vm_write_begin(struct vm_area_struct *vma)
+{
+}
+static inline void vm_write_begin_nested(struct vm_area_struct *vma,
+					 int subclass)
+{
+}
+static inline void vm_write_end(struct vm_area_struct *vma)
+{
+}
+static inline void vm_raw_write_begin(struct vm_area_struct *vma)
+{
+}
+static inline void vm_raw_write_end(struct vm_area_struct *vma)
+{
+}
+#endif /* __HAVE_ARCH_CALL_SPF */
+
 extern int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, int len,
 		unsigned int gup_flags);
 extern int access_remote_vm(struct mm_struct *mm, unsigned long addr,
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index d1e5062b0c00..6736feb8cb28 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -346,6 +346,9 @@ struct vm_area_struct {
 	struct mempolicy *vm_policy;	/* NUMA policy for the VMA */
 #endif
 	struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
+#ifdef __HAVE_ARCH_CALL_SPF
+	seqcount_t vm_sequence;
+#endif
 } __randomize_layout;
 
 struct core_thread {
diff --git a/mm/memory.c b/mm/memory.c
index 27f626c229f7..ac6822356a37 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1504,6 +1504,7 @@ void unmap_page_range(struct mmu_gather *tlb,
 	unsigned long next;
 
 	BUG_ON(addr >= end);
+	vm_write_begin(vma);
 	tlb_start_vma(tlb, vma);
 	pgd = pgd_offset(vma->vm_mm, addr);
 	do {
@@ -1513,6 +1514,7 @@ void unmap_page_range(struct mmu_gather *tlb,
 		next = zap_p4d_range(tlb, vma, pgd, addr, next, details);
 	} while (pgd++, addr = next, addr != end);
 	tlb_end_vma(tlb, vma);
+	vm_write_end(vma);
 }
 
 
diff --git a/mm/mmap.c b/mm/mmap.c
index 2f3971a051c6..ec896358fc5a 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -558,6 +558,10 @@ void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
 	else
 		mm->highest_vm_end = vm_end_gap(vma);
 
+#ifdef __HAVE_ARCH_CALL_SPF
+	seqcount_init(&vma->vm_sequence);
+#endif
+
 	/*
 	 * vma->vm_prev wasn't known when we followed the rbtree to find the
 	 * correct insertion point for that vma. As a result, we could not
@@ -692,6 +696,30 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
 	long adjust_next = 0;
 	int remove_next = 0;
 
+	/*
+	 * Why using vm_raw_write*() functions here to avoid lockdep's warning ?
+	 *
+	 * Locked is complaining about a theoretical lock dependency, involving
+	 * 3 locks:
+	 *   mapping->i_mmap_rwsem --> vma->vm_sequence --> fs_reclaim
+	 *
+	 * Here are the major path leading to this dependency :
+	 *  1. __vma_adjust() mmap_sem  -> vm_sequence -> i_mmap_rwsem
+	 *  2. move_vmap() mmap_sem -> vm_sequence -> fs_reclaim
+	 *  3. __alloc_pages_nodemask() fs_reclaim -> i_mmap_rwsem
+	 *  4. unmap_mapping_range() i_mmap_rwsem -> vm_sequence
+	 *
+	 * So there is no way to solve this easily, especially because in
+	 * unmap_mapping_range() the i_mmap_rwsem is grab while the impacted
+	 * VMAs are not yet known.
+	 * However, the way the vm_seq is used is guarantying that we will
+	 * never block on it since we just check for its value and never wait
+	 * for it to move, see vma_has_changed() and handle_speculative_fault().
+	 */
+	vm_raw_write_begin(vma);
+	if (next)
+		vm_raw_write_begin(next);
+
 	if (next && !insert) {
 		struct vm_area_struct *exporter = NULL, *importer = NULL;
 
@@ -903,6 +931,7 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
 		mm->map_count--;
 		mpol_put(vma_policy(next));
 		kmem_cache_free(vm_area_cachep, next);
+		vm_raw_write_end(next);
 		/*
 		 * In mprotect's case 6 (see comments on vma_merge),
 		 * we must remove another next too. It would clutter
@@ -916,6 +945,8 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
 			 * "vma->vm_next" gap must be updated.
 			 */
 			next = vma->vm_next;
+			if (next)
+				vm_raw_write_begin(next);
 		} else {
 			/*
 			 * For the scope of the comment "next" and
@@ -933,10 +964,9 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
 			remove_next = 1;
 			end = next->vm_end;
 			goto again;
-		}
-		else if (next)
+		} else if (next) {
 			vma_gap_update(next);
-		else {
+		} else {
 			/*
 			 * If remove_next == 2 we obviously can't
 			 * reach this path.
@@ -962,6 +992,10 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
 	if (insert && file)
 		uprobe_mmap(insert);
 
+	if (next && next != vma)
+		vm_raw_write_end(next);
+	vm_raw_write_end(vma);
+
 	validate_mm(mm);
 
 	return 0;
-- 
2.7.4