[BACKPORT] proc: less memory for /proc/*/map_files readdir

dentry name can be evaluated later, right before calling into VFS.

Also, spend less time under ->mmap_sem.

Link: http://lkml.kernel.org/r/20171110163034.GA2534@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Yahya Wessam <yahyawessam2002@gmail.com>

Author: Alexey Dobriyan

fs/proc/base.c

@@ -2277,9 +2277,9 @@ static int map_files_get_link(struct dentry *dentry, struct path *path)
 }
 
 struct map_files_info {
+	unsigned long	start;
+	unsigned long	end;
 	fmode_t		mode;
-	unsigned int	len;
-	unsigned char	name[4*sizeof(long)+2]; /* max: %lx-%lx\0 */
 };
 
 /*
@@ -2449,10 +2449,9 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
 			if (++pos <= ctx->pos)
 				continue;
 
+			info.start = vma->vm_start;
+			info.end = vma->vm_end;
 			info.mode = vma->vm_file->f_mode;
-			info.len = snprintf(info.name,
-					sizeof(info.name), "%lx-%lx",
-					vma->vm_start, vma->vm_end);
 			if (flex_array_put(fa, i++, &info, GFP_KERNEL))
 				BUG();
 		}
@@ -2461,9 +2460,13 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
 	mmput(mm);
 
 	for (i = 0; i < nr_files; i++) {
+		char buf[4 * sizeof(long) + 2];	/* max: %lx-%lx\0 */
+		unsigned int len;
+
 		p = flex_array_get(fa, i);
+		len = snprintf(buf, sizeof(buf), "%lx-%lx", p->start, p->end);
 		if (!proc_fill_cache(file, ctx,
-				      p->name, p->len,
+				      buf, len,
 				      proc_map_files_instantiate,
 				      task,
 				      (void *)(unsigned long)p->mode))