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>
---
 include/linux/mm_types.h |  9 ++++++++-
 mm/madvise.c             | 42 +++++++++++++++++++++++++++++++++-------
 2 files changed, 43 insertions(+), 8 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index bc029f3fca6a..32ac5dc5ebf3 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c

@@ -63,6 +63,27 @@ 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 = kzalloc(sizeof(*anon_name) + len + 1,
+			    GFP_KERNEL);
+	kref_init(&anon_name->kref);
+	strcpy(anon_name->name, name);
+
+	return anon_name;
+}