[PATCH 3/7] cxl: Keep track of mm struct associated with a context

[Andrew Donnellan]

On 02/02/17 04:30, Christophe Lombard wrote:
> The mm_struct corresponding to the current task is acquired each time
> an interrupt is raised. So to simplify the code, we only get the
> mm_struct when attaching an AFU context to the process.
> The mm_count reference is increased to ensure that the mm_struct can't
> be freed. The mm_struct will be released when the context is detached.
> The reference (use count) on the struct mm is not kept to avoid a
> circular dependency if the process mmaps its cxl mmio and forget to
> unmap before exiting.
>
> Signed-off-by: Christophe Lombard <clombard at linux.vnet.ibm.com>

One question below, otherwise this all looks good to me.

Reviewed-by: Andrew Donnellan <andrew.donnellan at au1.ibm.com>

> @@ -281,7 +212,6 @@ void cxl_handle_fault(struct work_struct *fault_work)
>  	if (!ctx->kernel) {
>
>  		mm = get_mem_context(ctx);
> -		/* indicates all the thread in task group have exited */
>  		if (mm == NULL) {
>  			pr_devel("%s: unable to get mm for pe=%d pid=%i\n",
>  				 __func__, ctx->pe, pid_nr(ctx->pid));

Is there still a case where mm can equal NULL?

> diff --git a/drivers/misc/cxl/file.c b/drivers/misc/cxl/file.c
> index 859959f..af6bd0e 100644
> --- a/drivers/misc/cxl/file.c
> +++ b/drivers/misc/cxl/file.c
> @@ -216,8 +216,16 @@ static long afu_ioctl_start_work(struct cxl_context *ctx,
>  	 * process is still accessible.
>  	 */
>  	ctx->pid = get_task_pid(current, PIDTYPE_PID);
> -	ctx->glpid = get_task_pid(current->group_leader, PIDTYPE_PID);
>
> +	/* acquire a reference to the task's mm */
> +	ctx->mm = get_task_mm(current);
> +
> +	/* ensure this mm_struct can't be freed */
> +	cxl_context_mm_count_get(ctx);
> +
> +	/* decrement the use count */
> +	if (ctx->mm)
> +		mmput(ctx->mm);

It took me a while to work out the difference between mm_users and 
mm_count... this looks fine.

-- 
Andrew Donnellan              OzLabs, ADL Canberra
andrew.donnellan at au1.ibm.com  IBM Australia Limited