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>

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 10a8c91c49d63b4f7bd61246286dae0b5c55d7f9..ddcb471b9cc95ab8ef9d3035acc0cb443fadcf0d 100644 (file)
--- 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;
 }

diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 4520c86f5cb40d387ebb415737179811ac3e7dd2..47f5235a097ac77edf9e28f93683c5015b04ff00 100644 (file)
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -178,9 +178,13 @@ struct kernfs_ops {
                        loff_t off);
 
        /*
-        * write() is bounced through kernel buffer and a write larger than
-        * PAGE_SIZE results in partial operation of PAGE_SIZE.
+        * write() is bounced through kernel buffer.  If atomic_write_len
+        * is not set, a write larger than PAGE_SIZE results in partial
+        * operations of PAGE_SIZE chunks.  If atomic_write_len is set,
+        * writes upto the specified size are executed atomically but
+        * larger ones are rejected with -E2BIG.
         */
+       size_t atomic_write_len;
        ssize_t (*write)(struct kernfs_open_file *of, char *buf, size_t bytes,
                         loff_t off);