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Commit 4d3773c4 authored by Tejun Heo's avatar Tejun Heo Committed by Greg Kroah-Hartman
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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: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent d35258ef

fs/kernfs/file.c

+31 −18
Original line number Diff line number Diff line
@@ -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;

}

include/linux/kernfs.h

+6 −2
Original line number Diff line number Diff line
@@ -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);