mm: Eliminate d_path_outlen() and further speed up show_map_vma()

d_path_outlen() isn't needed because we know that d_path() always
populates the given buffer backwards starting from the last byte; with
this, we can easily calculate the length of the generated string by
using the returned pointer from d_path() and the size of the buffer
given to d_path(). This eliminates the need for d_path_outlen() and
removes the bizarre strlen() usage, which makes things simpler and
faster. We also now avoid a memmove() when d_path() completely uses up
its provided buffer.

Signed-off-by: Sultan Alsawaf <[email protected]>
Change-Id: Ia52d062e7e12d9075a092f334ae49d9085fef03f

fs/d_path.c

@@ -249,9 +249,9 @@ static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
  *
  * "buflen" should be positive.
  */
-char *d_path_outlen(const struct path *path, char *buf, int *buflen)
+char *d_path(const struct path *path, char *buf, int buflen)
 {
-	char *res = buf + *buflen;
+	char *res = buf + buflen;
 	struct path root;
 	int error;
 
@@ -268,22 +268,17 @@ char *d_path_outlen(const struct path *path, char *buf, int *buflen)
 	 */
 	if (path->dentry->d_op && path->dentry->d_op->d_dname &&
 	    (!IS_ROOT(path->dentry) || path->dentry != path->mnt->mnt_root))
-		return path->dentry->d_op->d_dname(path->dentry, buf, *buflen);
+		return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
 
 	rcu_read_lock();
 	get_fs_root_rcu(current->fs, &root);
-	error = path_with_deleted(path, &root, &res, buflen);
+	error = path_with_deleted(path, &root, &res, &buflen);
 	rcu_read_unlock();
 
 	if (error < 0)
 		res = ERR_PTR(error);
 	return res;
 }
-
-char *d_path(const struct path *path, char *buf, int buflen)
-{
-	return d_path_outlen(path, buf, &buflen);
-}
 EXPORT_SYMBOL(d_path);
 
 /*

fs/proc/task_mmu.c

@@ -528,19 +528,16 @@ show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
 		 * program uses newlines in its paths then it can kick rocks.
 		 */
 		if (size > 1) {
-			const int inlen = size - 1;
-			int outlen = inlen;
 			char *p;
 
-			p = d_path_outlen(&file->f_path, buf, &outlen);
+			p = d_path(&file->f_path, buf, size);
 			if (!IS_ERR(p)) {
 				size_t len;
 
-				if (outlen != inlen)
-					len = inlen - outlen - 1;
-				else
-					len = strlen(p);
-				memmove(buf, p, len);
+				/* Minus one to exclude the NUL character */
+				len = size - (p - buf) - 1;
+				if (likely(p > buf))
+					memmove(buf, p, len);
 				buf[len] = '\n';
 				seq_commit(m, len + 1);
 				return;

include/linux/dcache.h

@@ -306,7 +306,6 @@ extern char *simple_dname(struct dentry *, char *, int);
 extern char *__d_path(const struct path *, const struct path *, char *, int);
 extern char *d_absolute_path(const struct path *, char *, int);
 extern char *d_path(const struct path *, char *, int);
-extern char *d_path_outlen(const struct path *, char *, int *);
 extern char *dentry_path_raw(struct dentry *, char *, int);
 extern char *dentry_path(struct dentry *, char *, int);