Re: [RFC Patch]: scsi: sysfs file cache_type not in sync with mode page

[Douglas Gilbert <dgilbert@interlog.com>]

On 14-06-06 06:55 PM, James Bottomley wrote:
> On Fri, 2014-06-06 at 17:14 -0400, Pai wrote:
>> Hi All,
>>
>> I noticed that the sysfs file cache_type is not is sync with the information in the mode page. If we change the WCE attribute in the mode page (sdparm --set=WCE /dev/sda  and sdparm --clear=WCE /dev/sda) it does not reflect this in the sysfs file.
>>
>> $ cat /sys/block/sda/device/scsi_disk/6\:0\:0\:0/cache_type
>> write back
>>
>> $ sdparm --clear=WCE /dev/sda
>>      /dev/sda: TOSHIBA   AL13SEB600        0101
>>
>> $ cat /sys/block/sda/device/scsi_disk/6\:0\:0\:0/cache_type
>> write back
>>
>> Ideally cache_type should have changed to 'write through' or 'none'. I have a small one line change that can fix this one. This patch applies against the latest branch "linux-3.15-rc8".
>>
>> Few things to note:
>> 1. revalidate_disk() also reads a whole bunch of other things from the mode page and I don't know if this will have any side effects. This call is made on store_cache_type as well, so I think it should be OK.
>> 2. This is extra work which is not needed in most cases (mode pages hardly change). Is there an event that we can subscribe to when the modpages change?
>>
>> Please review.
>>
>> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
>> index efcbcd1..5b65ccc 100644
>> --- a/drivers/scsi/sd.c
>> +++ b/drivers/scsi/sd.c
>> @@ -256,6 +256,7 @@ static ssize_t
>>   cache_type_show(struct device *dev, struct device_attribute *attr, char *buf)
>>   {
>>   	struct scsi_disk *sdkp = to_scsi_disk(dev);
>> +	revalidate_disk(sdkp->disk);
>>   	int ct = sdkp->RCD + 2*sdkp->WCE;
>>
>>   	return snprintf(buf, 40, "%s\n", sd_cache_types[ct]);
>
> The behaviour you describe is correct.  The cache type is probed once at
> init.  If you change the cache behind the back of the linux code, you
> have to expect that we won't see it.  We're definitely not doing a
> revalidate on every write just in case, so by extension we expect that
> to see it via sysfs either you change it through the provided interface
> (writing to the cache_type file) or you inform the kernel if you change
> it via other means by issuing the BLKRRPART ioctl.

Have a look at
   echo "write through" > /sys/class/scsi_disk/6\:0\:0\:0/cache_type

and/or
   echo "write back" > /sys/class/scsi_disk/6\:0\:0\:0/cache_type

and those strings can be prefixed by "temporary ". The explanation
and mapping to T10 terms is in drivers/scsi/sd.c cache_type_store(),
or at least should be.

Doug Gilbert