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@xxxxxxxxxx>
---
include/linux/mm_types.h | 9 ++++++++-
mm/madvise.c | 42 +++++++++++++++++++++++++++++++++-------
2 files changed, 43 insertions(+), 8 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 26a30f7a5228..a7361acf2921 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>
@@ -302,6 +303,12 @@ 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
@@ -353,7 +360,7 @@ 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 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;
+}
+
+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 +96,7 @@ 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 +105,44 @@ 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 void replace_vma_anon_name(struct vm_area_struct *vma, const char *name)
{
+ const char *anon_name;
+
if (!name) {
free_vma_anon_name(vma);
return;
}
- 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;
free_vma_anon_name(vma);
}
- vma->anon_name = kstrdup(name, GFP_KERNEL);
+ vma->anon_name = anon_vma_name_alloc(name);
}
/*
--
2.33.0.259.gc128427fd7-goog
