[PATCH v12 20/31] mm: introduce vma reference counter

[Laurent Dufour]

The final goal is to be able to use a VMA structure without holding the
mmap_sem and to be sure that the structure will not be freed in our back.

The lockless use of the VMA will be done through RCU protection and thus a
dedicated freeing service is required to manage it asynchronously.

As reported in a 2010's thread [1], this may impact file handling when a
file is still referenced while the mapping is no more there.  As the final
goal is to handle anonymous VMA in a speculative way and not file backed
mapping, we could close and free the file pointer in a synchronous way, as
soon as we are guaranteed to not use it without holding the mmap_sem. For
sanity reason, in a minimal effort, the vm_file file pointer is unset once
the file pointer is put.

[1] https://lore.kernel.org/linux-mm/20100104182429.833180340@chello.nl/

Signed-off-by: Laurent Dufour <ldufour at linux.ibm.com>
---
 include/linux/mm.h       |  4 ++++
 include/linux/mm_types.h |  3 +++
 mm/internal.h            | 27 +++++++++++++++++++++++++++
 mm/mmap.c                | 13 +++++++++----
 4 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f14b2c9ddfd4..f761a9c65c74 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -529,6 +529,9 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
 	vma->vm_mm = mm;
 	vma->vm_ops = &dummy_vm_ops;
 	INIT_LIST_HEAD(&vma->anon_vma_chain);
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+	atomic_set(&vma->vm_ref_count, 1);
+#endif
 }
 
 static inline void vma_set_anonymous(struct vm_area_struct *vma)
@@ -1418,6 +1421,7 @@ static inline void INIT_VMA(struct vm_area_struct *vma)
 	INIT_LIST_HEAD(&vma->anon_vma_chain);
 #ifdef CONFIG_SPECULATIVE_PAGE_FAULT
 	seqcount_init(&vma->vm_sequence);
+	atomic_set(&vma->vm_ref_count, 1);
 #endif
 }
 
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 24b3f8ce9e42..6a6159e11a3f 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -285,6 +285,9 @@ struct vm_area_struct {
 	/* linked list of VM areas per task, sorted by address */
 	struct vm_area_struct *vm_next, *vm_prev;
 
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+	atomic_t vm_ref_count;
+#endif
 	struct rb_node vm_rb;
 
 	/*
diff --git a/mm/internal.h b/mm/internal.h
index 9eeaf2b95166..302382bed406 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -40,6 +40,33 @@ void page_writeback_init(void);
 
 vm_fault_t do_swap_page(struct vm_fault *vmf);
 
+
+extern void __free_vma(struct vm_area_struct *vma);
+
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+static inline void get_vma(struct vm_area_struct *vma)
+{
+	atomic_inc(&vma->vm_ref_count);
+}
+
+static inline void put_vma(struct vm_area_struct *vma)
+{
+	if (atomic_dec_and_test(&vma->vm_ref_count))
+		__free_vma(vma);
+}
+
+#else
+
+static inline void get_vma(struct vm_area_struct *vma)
+{
+}
+
+static inline void put_vma(struct vm_area_struct *vma)
+{
+	__free_vma(vma);
+}
+#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
+
 void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
 		unsigned long floor, unsigned long ceiling);
 
diff --git a/mm/mmap.c b/mm/mmap.c
index f7f6027a7dff..c106440dcae7 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -188,6 +188,12 @@ static inline void mm_write_sequnlock(struct mm_struct *mm)
 }
 #endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
 
+void __free_vma(struct vm_area_struct *vma)
+{
+	mpol_put(vma_policy(vma));
+	vm_area_free(vma);
+}
+
 /*
  * Close a vm structure and free it, returning the next.
  */
@@ -200,8 +206,8 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
 		vma->vm_ops->close(vma);
 	if (vma->vm_file)
 		fput(vma->vm_file);
-	mpol_put(vma_policy(vma));
-	vm_area_free(vma);
+	vma->vm_file = NULL;
+	put_vma(vma);
 	return next;
 }
 
@@ -990,8 +996,7 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
 		if (next->anon_vma)
 			anon_vma_merge(vma, next);
 		mm->map_count--;
-		mpol_put(vma_policy(next));
-		vm_area_free(next);
+		put_vma(next);
 		/*
 		 * In mprotect's case 6 (see comments on vma_merge),
 		 * we must remove another next too. It would clutter
-- 
2.21.0