[PATCH v9 3/3] mm: add anonymous vma name refcounting

[Suren Baghdasaryan <surenb@google.com>]

While forking a process with high number (64K) of named anonymous vmas the
overhead caused by strdup() is noticeable. Experiments with ARM64 Android
device show up to 40% performance regression when forking a process with
64k unpopulated anonymous vmas using the max name lengths vs the same
process with the same number of anonymous vmas having no name.
Introduce anon_vma_name refcounted structure to avoid the overhead of
copying vma names during fork() and when splitting named anonymous vmas.
When a vma is duplicated, instead of copying the name we increment the
refcount of this structure. Multiple vmas can point to the same
anon_vma_name as long as they increment the refcount. The name member of
anon_vma_name structure is assigned at structure allocation time and is
never changed. If vma name changes then the refcount of the original
structure is dropped, a new anon_vma_name structure is allocated
to hold the new name and the vma pointer is updated to point to the new
structure.
With this approach the fork() performance regressions is reduced 3-4x
times and with usecases using more reasonable number of VMAs (a few
thousand) the regressions is not measurable.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
previous version including cover letter with test results is at:
https://lore.kernel.org/linux-mm/20210827191858.2037087-1-surenb@google.com/

changes in v9
- Replaced kzalloc with kmalloc in anon_vma_name_alloc, per Rolf Eike Beer

 include/linux/mm_types.h |  9 ++++++++-
 mm/madvise.c             | 43 +++++++++++++++++++++++++++++++++-------
 2 files changed, 44 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 968a1d0463d8..7feb43daee6c 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -5,6 +5,7 @@
 #include <linux/mm_types_task.h>
 
 #include <linux/auxvec.h>
+#include <linux/kref.h>
 #include <linux/list.h>
 #include <linux/spinlock.h>
 #include <linux/rbtree.h>
@@ -310,6 +311,12 @@ struct vm_userfaultfd_ctx {
 struct vm_userfaultfd_ctx {};
 #endif /* CONFIG_USERFAULTFD */
 
+struct anon_vma_name {
+	struct kref kref;
+	/* The name needs to be at the end because it is dynamically sized. */
+	char name[];
+};
+
 /*
  * This struct describes a virtual memory area. There is one of these
  * per VM-area/task. A VM area is any part of the process virtual memory
@@ -361,7 +368,7 @@ struct vm_area_struct {
 			unsigned long rb_subtree_last;
 		} shared;
 		/* Serialized by mmap_sem. */
-		char *anon_name;
+		struct anon_vma_name *anon_name;
 	};
 
 	/*
diff --git a/mm/madvise.c b/mm/madvise.c
index 0c6d0f64d432..adc53edd3fe7 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -63,6 +63,28 @@ static int madvise_need_mmap_write(int behavior)
 	}
 }
 
+static struct anon_vma_name *anon_vma_name_alloc(const char *name)
+{
+	struct anon_vma_name *anon_name;
+	size_t len = strlen(name);
+
+	/* Add 1 for NUL terminator at the end of the anon_name->name */
+	anon_name = kmalloc(sizeof(*anon_name) + len + 1, GFP_KERNEL);
+	if (anon_name) {
+		kref_init(&anon_name->kref);
+		strcpy(anon_name->name, name);
+	}
+
+	return anon_name;
+}
+
+static void vma_anon_name_free(struct kref *kref)
+{
+	struct anon_vma_name *anon_name =
+			container_of(kref, struct anon_vma_name, kref);
+	kfree(anon_name);
+}
+
 static inline bool has_vma_anon_name(struct vm_area_struct *vma)
 {
 	return !vma->vm_file && vma->anon_name;
@@ -75,7 +97,7 @@ const char *vma_anon_name(struct vm_area_struct *vma)
 
 	mmap_assert_locked(vma->vm_mm);
 
-	return vma->anon_name;
+	return vma->anon_name->name;
 }
 
 void dup_vma_anon_name(struct vm_area_struct *orig_vma,
@@ -84,37 +106,44 @@ void dup_vma_anon_name(struct vm_area_struct *orig_vma,
 	if (!has_vma_anon_name(orig_vma))
 		return;
 
-	new_vma->anon_name = kstrdup(orig_vma->anon_name, GFP_KERNEL);
+	kref_get(&orig_vma->anon_name->kref);
+	new_vma->anon_name = orig_vma->anon_name;
 }
 
 void free_vma_anon_name(struct vm_area_struct *vma)
 {
+	struct anon_vma_name *anon_name;
+
 	if (!has_vma_anon_name(vma))
 		return;
 
-	kfree(vma->anon_name);
+	anon_name = vma->anon_name;
 	vma->anon_name = NULL;
+	kref_put(&anon_name->kref, vma_anon_name_free);
 }
 
 /* mmap_lock should be write-locked */
 static int replace_vma_anon_name(struct vm_area_struct *vma, const char *name)
 {
+	const char *anon_name;
+
 	if (!name) {
 		free_vma_anon_name(vma);
 		return 0;
 	}
 
-	if (vma->anon_name) {
+	anon_name = vma_anon_name(vma);
+	if (anon_name) {
 		/* Should never happen, to dup use dup_vma_anon_name() */
-		WARN_ON(vma->anon_name == name);
+		WARN_ON(anon_name == name);
 
 		/* Same name, nothing to do here */
-		if (!strcmp(name, vma->anon_name))
+		if (!strcmp(name, anon_name))
 			return 0;
 
 		free_vma_anon_name(vma);
 	}
-	vma->anon_name = kstrdup(name, GFP_KERNEL);
+	vma->anon_name = anon_vma_name_alloc(name);
 	if (!vma->anon_name)
 		return -ENOMEM;
 
-- 
2.33.0.153.gba50c8fa24-goog