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>
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@ -252,19 +252,9 @@ static ssize_t kernfs_fop_write(struct file *file, const char __user *user_buf,
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size_t count, loff_t *ppos)
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{
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struct kernfs_open_file *of = kernfs_of(file);
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ssize_t len = min_t(size_t, count, PAGE_SIZE);
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const struct kernfs_ops *ops;
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char *buf;
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buf = kmalloc(len + 1, GFP_KERNEL);
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if (!buf)
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return -ENOMEM;
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if (copy_from_user(buf, user_buf, len)) {
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len = -EFAULT;
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goto out_free;
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}
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buf[len] = '\0'; /* guarantee string termination */
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char *buf = NULL;
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ssize_t len;
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/*
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* @of->mutex nests outside active ref and is just to ensure that
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@ -273,22 +263,45 @@ static ssize_t kernfs_fop_write(struct file *file, const char __user *user_buf,
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mutex_lock(&of->mutex);
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if (!kernfs_get_active(of->kn)) {
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mutex_unlock(&of->mutex);
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len = -ENODEV;
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goto out_free;
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return -ENODEV;
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}
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ops = kernfs_ops(of->kn);
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if (ops->write)
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len = ops->write(of, buf, len, *ppos);
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else
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if (!ops->write) {
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len = -EINVAL;
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goto out_unlock;
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}
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if (ops->atomic_write_len) {
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len = count;
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if (len > ops->atomic_write_len) {
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len = -E2BIG;
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goto out_unlock;
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}
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} else {
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len = min_t(size_t, count, PAGE_SIZE);
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}
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buf = kmalloc(len + 1, GFP_KERNEL);
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if (!buf) {
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len = -ENOMEM;
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goto out_unlock;
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}
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if (copy_from_user(buf, user_buf, len)) {
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len = -EFAULT;
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goto out_unlock;
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}
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buf[len] = '\0'; /* guarantee string termination */
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len = ops->write(of, buf, len, *ppos);
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out_unlock:
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kernfs_put_active(of->kn);
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mutex_unlock(&of->mutex);
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if (len > 0)
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*ppos += len;
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out_free:
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kfree(buf);
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return len;
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}
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@ -178,9 +178,13 @@ struct kernfs_ops {
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loff_t off);
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/*
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* write() is bounced through kernel buffer and a write larger than
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* PAGE_SIZE results in partial operation of PAGE_SIZE.
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* write() is bounced through kernel buffer. If atomic_write_len
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* is not set, a write larger than PAGE_SIZE results in partial
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* operations of PAGE_SIZE chunks. If atomic_write_len is set,
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* writes upto the specified size are executed atomically but
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* larger ones are rejected with -E2BIG.
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*/
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size_t atomic_write_len;
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ssize_t (*write)(struct kernfs_open_file *of, char *buf, size_t bytes,
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loff_t off);
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