Re: block: disallow changing max_sectors_kb on a request stacking device

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Oct 28 2016 at  3:45pm -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> Otherwise users can easily shoot themselves in the foot by creating the
> situation where the top-level stacked device (e.g. DM multipath) has a
> larger max_sectors_kb than the underlying device(s).  Which will
> certainly lead to IO errors due to the "over max size limit" check in
> blk_cloned_rq_check_limits().
> 
> Use of WARN_ON_ONCE() gives users visibility into which application
> attempted to perform this unsupported max_sectors_kb change.
> 
> This is a crude, yet effective, solution that forces the use of system
> software (e.g. udev rules or multipathd) to tweak max_sectors_kb of the
> underlying devices _before_ a layer like DM multipath is layered ontop.
> 
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> ---
>  block/blk-sysfs.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 9cc8d7c..cce43d7 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -199,8 +199,12 @@ queue_max_sectors_store(struct request_queue *q, const char *page, size_t count)
>  	unsigned long max_sectors_kb,
>  		max_hw_sectors_kb = queue_max_hw_sectors(q) >> 1,
>  			page_kb = 1 << (PAGE_SHIFT - 10);
> -	ssize_t ret = queue_var_store(&max_sectors_kb, page, count);
> +	ssize_t ret;
> +
> +	if (WARN_ON_ONCE(blk_queue_stackable(q)))
> +		return -EINVAL;
>  
> +	ret = queue_var_store(&max_sectors_kb, page, count);
>  	if (ret < 0)
>  		return ret;
>  
> -- 
> 2.8.4 (Apple Git-73)

Jens,

Never heard from you on this patch.  Would you like me to resend without
the WARN_ON_ONCE?  (Failing is enough, the userspace code that attempts
to set max_sectors_kb on DM multipath will get the user's attention)