[Devel] Re: [RFC/PATCH 2/8]: CGroup Files: Add a cgroup write_string control file method

[Andrew Morton]

On Mon, 12 May 2008 23:37:09 -0700
menage at google.com wrote:

> This patch adds a write_string() method for cgroups control files. The
> semantics are that a buffer is copied from userspace to kernelspace
> and the handler function invoked on that buffer.  Any control group
> locking is done after the copy from userspace has occurred. The buffer
> is guaranteed to be nul-terminated, and no longer than max_write_len
> (defaulting to 64 bytes if unspecified). Later patches will convert
> existing raw file write handlers in control group subsystems to use
> this method.
> 

nits:

> 
> ---
>  include/linux/cgroup.h |   10 ++++++++++
>  kernel/cgroup.c        |    5 ++++-
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> Index: cgroup-2.6.25-mm1/include/linux/cgroup.h
> ===================================================================
> --- cgroup-2.6.25-mm1.orig/include/linux/cgroup.h
> +++ cgroup-2.6.25-mm1/include/linux/cgroup.h
> @@ -281,6 +281,10 @@ struct cftype {
>  	 */
>  	int lockmode;
>  
> +	/* If non-zero, defines the maximum length of string that can
> +	 * be passed to write_string; defaults to 64 */
> +	int max_write_len;

would size_t be a more appropriate type?

>  	int (*open) (struct inode *inode, struct file *file);
>  	ssize_t (*read) (struct cgroup *cgrp, struct cftype *cft,
>  			 struct file *file,
> @@ -323,6 +327,12 @@ struct cftype {
>  	 * write_s64() is a signed version of write_u64()
>  	 */
>  	int (*write_s64) (struct cgroup *cgrp, struct cftype *cft, s64 val);

s/) (/)(/ would be more conventional.

> +	/*
> +	 * write_string() is passed a nul-terminated kernelspace
> +	 * buffer of maximum length determined by max_write_len
> +	 */
> +	int (*write_string) (struct cgroup *cgrp, struct cftype *cft,
> +			     char *buffer);

Should these return size_t?

>  	/*
>  	 * trigger() callback can be used to get some kick from the
> Index: cgroup-2.6.25-mm1/kernel/cgroup.c
> ===================================================================
> --- cgroup-2.6.25-mm1.orig/kernel/cgroup.c
> +++ cgroup-2.6.25-mm1/kernel/cgroup.c
> @@ -1461,7 +1461,7 @@ static ssize_t cgroup_file_write(struct 
>  	ssize_t retval;
>  	char static_buffer[64];
>  	char *buffer = static_buffer;
> -	ssize_t max_bytes = sizeof(static_buffer) - 1;
> +	ssize_t max_bytes =  cft->max_write_len ?: sizeof(static_buffer) - 1;

A blank line between end-of-locals and start-of-code is conventional
and, IMO, easier on the eye.

Does gcc actually generate better code with that x?:y thing?  Because
it always makes me pause and scratch my head.

>  	if (!cft->write && !cft->trigger) {
>  		if (!nbytes)
>  			return -EINVAL;
> @@ -1489,6 +1489,8 @@ static ssize_t cgroup_file_write(struct 
>  		retval = cft->write(cgrp, cft, file, userbuf, nbytes, ppos);
>  	else if (cft->write_u64 || cft->write_s64)
>  		retval = cgroup_write_X64(cgrp, cft, buffer);
> +	else if (cft->write_string)
> +		retval = cft->write_string(cgrp, cft, buffer);
>  	else if (cft->trigger)
>  		retval = cft->trigger(cgrp, (unsigned int)cft->private);
>  	else
> @@ -1651,6 +1653,7 @@ static struct file_operations cgroup_seq
>  	.read = seq_read,
>  	.llseek = seq_lseek,
>  	.release = cgroup_seqfile_release,
> +	.write = cgroup_file_write,
>  };
>  
>  static int cgroup_file_open(struct inode *inode, struct file *file)

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