[SCSI] sd: fix array cache flushing bug causing performance problems

Some arrays synchronize their full non volatile cache when the sd driver sends
a SYNCHRONIZE CACHE command. Unfortunately, they can have Terrabytes of this
and we send a SYNCHRONIZE CACHE for every barrier if an array reports it has a
writeback cache. This leads to massive slowdowns on journalled filesystems.

The fix is to allow userspace to turn off the writeback cache setting as a
temporary measure (i.e. without doing the MODE SELECT to write it back to the
device), so even though the device reported it has a writeback cache, the
user, knowing that the cache is non volatile and all they care about is
filesystem correctness, can turn that bit off in the kernel and avoid the
performance ruinous (and safety irrelevant) SYNCHRONIZE CACHE commands.

The way you do this is add a 'temporary' prefix when performing the usual
cache setting operations, so

echo temporary write through > /sys/class/scsi_disk/<disk>/cache_type

Reported-by: Ric Wheeler <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: James Bottomley <[e-mail]>

drivers/scsi/sd.c

···
 	char *buffer_data;
 	struct scsi_mode_data data;
 	struct scsi_sense_hdr sshdr;
+	const char *temp = "temporary ";
 	int len;
 
 	if (sdp->type != TYPE_DISK)
···
 		 * can do it, but there's probably so many exceptions
 		 * it's not worth the risk */
 		return -EINVAL;
+
+	if (strncmp(buf, temp, sizeof(temp) - 1) == 0) {
+		buf += sizeof(temp) - 1;
+		sdkp->cache_override = 1;
+	} else {
+		sdkp->cache_override = 0;
+	}
 
 	for (i = 0; i < ARRAY_SIZE(sd_cache_types); i++) {
 		len = strlen(sd_cache_types[i]);
···
 		return -EINVAL;
 	rcd = ct & 0x01 ? 1 : 0;
 	wce = ct & 0x02 ? 1 : 0;
+
+	if (sdkp->cache_override) {
+		sdkp->WCE = wce;
+		sdkp->RCD = rcd;
+		return count;
+	}
+
 	if (scsi_mode_sense(sdp, 0x08, 8, buffer, sizeof(buffer), SD_TIMEOUT,
 			    SD_MAX_RETRIES, &data, NULL))
 		return -EINVAL;
···
 	int old_rcd = sdkp->RCD;
 	int old_dpofua = sdkp->DPOFUA;
 
+
+	if (sdkp->cache_override)
+		return;
+
 	first_len = 4;
 	if (sdp->skip_ms_page_8) {
 		if (sdp->type == TYPE_RBC)
···
 	sdkp->capacity = 0;
 	sdkp->media_present = 1;
 	sdkp->write_prot = 0;
+	sdkp->cache_override = 0;
 	sdkp->WCE = 0;
 	sdkp->RCD = 0;
 	sdkp->ATO = 0;

drivers/scsi/sd.h

···
 	u8		protection_type;/* Data Integrity Field */
 	u8		provisioning_mode;
 	unsigned	ATO : 1;	/* state of disk ATO bit */
+	unsigned	cache_override : 1; /* temp override of WCE,RCD */
 	unsigned	WCE : 1;	/* state of disk WCE bit */
 	unsigned	RCD : 1;	/* state of disk RCD bit, unused */
 	unsigned	DPOFUA : 1;	/* state of disk DPOFUA bit */