kernfs: implement kernfs_ops->atomic_write_len

A write to a kernfs_node is buffered through a kernel buffer.  Writes
<= PAGE_SIZE are performed atomically, while larger ones are executed
in PAGE_SIZE chunks.  While this is enough for sysfs, cgroup which is
scheduled to be converted to use kernfs needs a bit more control over
it.

This patch adds kernfs_ops->atomic_write_len.  If not set (zero), the
behavior stays the same.  If set, writes upto the size are executed
atomically and larger writes are rejected with -E2BIG.

A different implementation strategy would be allowing configuring
chunking size while making the original write size available to the
write method; however, such strategy, while being more complicated,
doesn't really buy anything.  If the write implementation has to
handle chunking, the specific chunk size shouldn't matter all that
much.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 31 insertions, 18 deletions

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 10a8c91c49d6..ddcb471b9cc9 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -252,19 +252,9 @@ static ssize_t kernfs_fop_write(struct file *file, const char __user *user_buf,
                                 size_t count, loff_t *ppos)
 {
         struct kernfs_open_file *of = kernfs_of(file);
-        ssize_t len = min_t(size_t, count, PAGE_SIZE);
         const struct kernfs_ops *ops;
-        char *buf;
-
-        buf = kmalloc(len + 1, GFP_KERNEL);
-        if (!buf)
-                return -ENOMEM;
-
-        if (copy_from_user(buf, user_buf, len)) {
-                len = -EFAULT;
-                goto out_free;
-        }
-        buf[len] = '\0';        /* guarantee string termination */
+        char *buf = NULL;
+        ssize_t len;
 
         /*
          * @of->mutex nests outside active ref and is just to ensure that
@@ -273,22 +263,45 @@ static ssize_t kernfs_fop_write(struct file *file, const char __user *user_buf,
         mutex_lock(&of->mutex);
         if (!kernfs_get_active(of->kn)) {
                 mutex_unlock(&of->mutex);
-                len = -ENODEV;
-                goto out_free;
+                return -ENODEV;
         }
 
         ops = kernfs_ops(of->kn);
-        if (ops->write)
-                len = ops->write(of, buf, len, *ppos);
-        else
+        if (!ops->write) {
                 len = -EINVAL;
+                goto out_unlock;
+        }
+
+        if (ops->atomic_write_len) {
+                len = count;
+                if (len > ops->atomic_write_len) {
+                        len = -E2BIG;
+                        goto out_unlock;
+                }
+        } else {
+                len = min_t(size_t, count, PAGE_SIZE);
+        }
+
+        buf = kmalloc(len + 1, GFP_KERNEL);
+        if (!buf) {
+                len = -ENOMEM;
+                goto out_unlock;
+        }
 
+        if (copy_from_user(buf, user_buf, len)) {
+                len = -EFAULT;
+                goto out_unlock;
+        }
+        buf[len] = '\0';        /* guarantee string termination */
+
+        len = ops->write(of, buf, len, *ppos);
+out_unlock:
         kernfs_put_active(of->kn);
         mutex_unlock(&of->mutex);
 
         if (len > 0)
                 *ppos += len;
-out_free:
+
         kfree(buf);
         return len;
 }