Re: [PATCH 2/2] sysfs/kernfs: make read requests on pre-alloc files use the buffer.
From: Tejun Heo
Date: Thu Oct 09 2014 - 09:52:54 EST
On Thu, Oct 09, 2014 at 10:57:06AM +1100, NeilBrown wrote:
> To match the previous patch which used the pre-alloc buffer for
> writes, this patch causes reads to use the same buffer.
> This is not strictly necessary as the current seq_read() will allocate
> on first read, so user-space can trigger the required pre-alloc. But
> consistency is valuable.
> The read function is somewhat simpler than seq_read() and, for example,
> does not support reading from an offset into the file: reads must be
> at the start of the file.
> As seq_read() does not use the prealloc buffer, ->seq_show is
> incompatible with ->prealloc and caused an EINVAL return from open().
> sysfs code which calls into kernfs always chooses the correct function.
> As the buffer is shared with writes and other reads, the mutex is
> extended to cover the copy_to_user.
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
Reviewed-by: Tejun Heo <tj@xxxxxxxxxx>
> @@ -690,6 +694,12 @@ static int kernfs_fop_open(struct inode *inode, struct file *file)
> of->atomic_write_len = ops->atomic_write_len;
> + error = -EINVAL;
> + if (ops->prealloc && ops->seq_show)
> + /* ->seq_show is incompatible with ->prealloc,
> + * ->read must be used instead.
> + */
> + goto err_free;
Let's please use fully-winged comments. If it looks weird inside the
if block, it can just be located right on top, I think. Also,
wouldn't it be better if the comment explained the reason for the
incompatibility? Along the same line, I think it'd be better if this
is also explicitly explained where ->prealloc is defined.
> +/* kernfs read callback for regular sysfs files with pre-alloc */
> +static ssize_t sysfs_kf_read(struct kernfs_open_file *of, char *buf,
> + size_t count, loff_t pos)
> + const struct sysfs_ops *ops = sysfs_file_ops(of->kn);
> + struct kobject *kobj = of->kn->parent->priv;
> + if (pos || buf != of->prealloc_buf)
> + /* If buf != of->prealloc_buf, we don't know how
> + * large it is, so cannot safely pass it to ->show
> + */
> + return 0;
> + return ops->show(kobj, of->kn->priv, buf);
Ditto on the comment formatting also shouldn't the latter condition be
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