Re: [PATCH v2 1/1] tools/hotplug: Scan xenstore once when attaching shared images files

[Ian Campbell <ian.campbell@citrix.com>]

On Thu, 2015-10-01 at 13:07 -0600, Mike Latimer wrote:
> During the attachment of a loopback mounted image file, the mode of all
> curent instances of this device already attached to other domains must be
> checked. This requires finding all loopback devices pointing to the inode
> of the shared image file, and then comparing the major and minor number of
> these devices to the major and minor number of every vbd device found in the
> xenstore database.
> 
> Prior to this patch, the entire xenstore database is walked for every instance
> of every loopback device pointing to the same shared image file. This process
> causes the block attachment process to becomes exponentially slower with every
> additional attachment of a shared image.
> 
> Rather than scanning all of xenstore for every instance of a shared loopback
> device, this patch creates a list of the major and minor numbers from all
> matching loopback devices. After generating this list, Xenstore is walked
> once, and major and minor numbers from every vbd are checked against the list.
> If a match is found, the mode of that vbd is checked for compatibility with
> the mode of the device being attached.
> 
> Signed-off-by: Mike Latimer <mlatimer@suse.com>

This looks good to me apart from one thing which is that these:
[...]
>        then
> -        echo 'local'
> +        echo "local $d"
> 
[...]
>            then
> -            echo 'guest'
> +            echo "guest $d"
[...]
>              then
> -              echo 'guest'
> +              echo "guest $d"

Are invalidating this comment on the check_sharing function regarding what
it returns:

# Prints one of
#
#    'local': the device may not be used because it is mounted in the current
#             (i.e. the privileged domain) in a way incompatible with the
#             requested mode;
#    'guest': the device may not be used because it already mounted by a guest
#             in a way incompatible with the requested mode; or
#    'ok':    the device may be used.

Ian.