mm: add a field to store names for private anonymous memory

Userspace processes often have multiple allocators that each do
anonymous mmaps to get memory.  When examining memory usage of
individual processes or systems as a whole, it is useful to be
able to break down the various heaps that were allocated by
each layer and examine their size, RSS, and physical memory
usage.

This patch adds a user pointer to the shared union in
vm_area_struct that points to a null terminated string inside
the user process containing a name for the vma.  vmas that
point to the same address will be merged, but vmas that
point to equivalent strings at different addresses will
not be merged.

Userspace can set the name for a region of memory by calling
prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, start, len, (unsigned long)name);
Setting the name to NULL clears it.

The names of named anonymous vmas are shown in /proc/pid/maps
as [anon:<name>] and in /proc/pid/smaps in a new "Name" field
that is only present for named vmas.  If the userspace pointer
is no longer valid all or part of the name will be replaced
with "<fault>".

The idea to store a userspace pointer to reduce the complexity
within mm (at the expense of the complexity of reading
/proc/pid/mem) came from Dave Hansen.  This results in no
runtime overhead in the mm subsystem other than comparing
the anon_name pointers when considering vma merging.  The pointer
is stored in a union with fieds that are only used on file-backed
mappings, so it does not increase memory usage.

Change-Id: I6ed36e1bcac7a29132fde1667ac0f62dcda69e44

3 files changed, 19 insertions, 1 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ded184f..fa535c0 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1407,7 +1407,7 @@ extern int vma_adjust(struct vm_area_struct *vma, unsigned long start,
 extern struct vm_area_struct *vma_merge(struct mm_struct *,
 	struct vm_area_struct *prev, unsigned long addr, unsigned long end,
 	unsigned long vm_flags, struct anon_vma *, struct file *, pgoff_t,
-	struct mempolicy *);
+	struct mempolicy *, const char __user *);
 extern struct anon_vma *find_mergeable_anon_vma(struct vm_area_struct *);
 extern int split_vma(struct mm_struct *,
 	struct vm_area_struct *, unsigned long addr, int new_below);
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index b30298a..8663540 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -162,6 +162,10 @@ struct vm_area_struct {
 	 * linkage into the address_space->i_mmap prio tree, or
 	 * linkage to the list of like vmas hanging off its node, or
 	 * linkage of vma in the address_space->i_mmap_nonlinear list.
+	 *
+	 * For private anonymous mappings, a pointer to a null terminated string
+	 * in the user process containing the name given to the vma, or NULL
+	 * if unnamed.
 	 */
 	union {
 		struct {
@@ -171,6 +175,7 @@ struct vm_area_struct {
 		} vm_set;
 
 		struct raw_prio_tree_node prio_tree_node;
+		const char __user *anon_name;
 	} shared;
 
 	/*
@@ -342,4 +347,14 @@ static inline cpumask_t *mm_cpumask(struct mm_struct *mm)
 	return mm->cpu_vm_mask_var;
 }
 
+
+/* Return the name for an anonymous mapping or NULL for a file-backed mapping */
+static inline const char __user *vma_get_anon_name(struct vm_area_struct *vma)
+{
+	if (vma->vm_file)
+		return NULL;
+
+	return vma->shared.anon_name;
+}
+
 #endif /* _LINUX_MM_TYPES_H */
diff --git a/include/linux/prctl.h b/include/linux/prctl.h
index 1c0b14a..97d5e23 100644
--- a/include/linux/prctl.h
+++ b/include/linux/prctl.h
@@ -108,4 +108,7 @@
  */
 #define PR_SET_TIMERSLACK_PID 41
 
+#define PR_SET_VMA		0x53564d41
+# define PR_SET_VMA_ANON_NAME		0
+
 #endif /* _LINUX_PRCTL_H */