mm: add anonymous vma name refcounting

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.

Link: https://lkml.kernel.org/r/20211019215511.3771969-3-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Colin Cross <ccross@google.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jan Glauber <jan.glauber@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rob Landley <rob@landley.net>
Cc: "Serge E. Hallyn" <serge.hallyn@ubuntu.com>
Cc: Shaohua Li <shli@fusionio.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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>
@@ -386,6 +387,12 @@ 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
@@ -437,7 +444,7 @@ struct vm_area_struct {
 			unsigned long rb_subtree_last;
 		} shared;
 		/* Serialized by mmap_sem. */
-		char *anon_name;
+		struct anon_vma_name *anon_name;
 	};
 
 	/*

mm/madvise.c

@@ -64,6 +64,29 @@ static int madvise_need_mmap_write(int behavior)
 }
 
 #ifdef CONFIG_ANON_VMA_NAME
+static struct anon_vma_name *anon_vma_name_alloc(const char *name)
+{
+	struct anon_vma_name *anon_name;
+	size_t count;
+
+	/* Add 1 for NUL terminator at the end of the anon_name->name */
+	count = strlen(name) + 1;
+	anon_name = kmalloc(struct_size(anon_name, name, count), GFP_KERNEL);
+	if (anon_name) {
+		kref_init(&anon_name->kref);
+		memcpy(anon_name->name, name, count);
+	}
+
+	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;
@@ -76,7 +99,7 @@ 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,
@@ -85,34 +108,41 @@ 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) {
 		/* 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;