[v3,02/15] scsi_debug: add doublestore option

Commit Message

Douglas Gilbert Feb. 20, 2020, 8:08 p.m. UTC

The scsi_debug driver has always been restricted to using one
(or none) ramdisk image for its storage. This means that thousands
of scsi_debug devices can be created without exhausting the host
machine's RAM. The downside is that all scsi_debug devices share
the same ramdisk image. This option doubles the amount of ramdisk
storage space with the first, third, fifth (etc) created
scsi_debug devices using the first ramdisk image while the second,
fourth, sixth (etc) created scsi_debug devices using the second
ramdisk image.

The reason for doing this is to check that (partial) disk to disk
copies based on scsi_debug devices have actually worked properly.
As an example: assume /dev/sdb and /dev/sg1 are the same
scsi_debug device, while /dev/sdc and /dev/sg2 are also the
same scsi_debug device. With doublestore=1 they will have
different ramdisk images. Then the following pseudocode could
be executed to check the if sgh_dd copy worked:
    dd if=/dev/urandom of=/dev/sdb
    sgh_dd if=/dev/sg1 of=/dev/sg2 [plus option(s) to test]
    cmp /dev/sdb /dev/sdc

If the cmp fails then the copy has failed (or some other
mechanism wrote to /dev/sdb or /dev/sdc in the interim).

Signed-off-by: Douglas Gilbert <[e-mail]>
---
 drivers/scsi/scsi_debug.c | 264 +++++++++++++++++++++++++++-----------
 1 file changed, 186 insertions(+), 78 deletions(-)

Comments

Hannes Reinecke Feb. 21, 2020, 9:46 a.m. UTC | #1

On 2/20/20 9:08 PM, Douglas Gilbert wrote:
> The scsi_debug driver has always been restricted to using one
> (or none) ramdisk image for its storage. This means that thousands
> of scsi_debug devices can be created without exhausting the host
> machine's RAM. The downside is that all scsi_debug devices share
> the same ramdisk image. This option doubles the amount of ramdisk
> storage space with the first, third, fifth (etc) created
> scsi_debug devices using the first ramdisk image while the second,
> fourth, sixth (etc) created scsi_debug devices using the second
> ramdisk image.
> 
> The reason for doing this is to check that (partial) disk to disk
> copies based on scsi_debug devices have actually worked properly.
> As an example: assume /dev/sdb and /dev/sg1 are the same
> scsi_debug device, while /dev/sdc and /dev/sg2 are also the
> same scsi_debug device. With doublestore=1 they will have
> different ramdisk images. Then the following pseudocode could
> be executed to check the if sgh_dd copy worked:
>     dd if=/dev/urandom of=/dev/sdb
>     sgh_dd if=/dev/sg1 of=/dev/sg2 [plus option(s) to test]
>     cmp /dev/sdb /dev/sdc
> 
> If the cmp fails then the copy has failed (or some other
> mechanism wrote to /dev/sdb or /dev/sdc in the interim).
> 
> Signed-off-by: Douglas Gilbert <[e-mail]>
> ---
>  drivers/scsi/scsi_debug.c | 264 +++++++++++++++++++++++++++-----------
>  1 file changed, 186 insertions(+), 78 deletions(-)
> 
Nice use-case, but really should be documented somewhere.
Otherwise:

Reviewed-by: Hannes Reinecke <[e-mail]>

Cheers,

Hannes

Douglas Gilbert Feb. 21, 2020, 3:37 p.m. UTC | #2

On 2020-02-21 4:46 a.m., Hannes Reinecke wrote:
> On 2/20/20 9:08 PM, Douglas Gilbert wrote:
>> The scsi_debug driver has always been restricted to using one
>> (or none) ramdisk image for its storage. This means that thousands
>> of scsi_debug devices can be created without exhausting the host
>> machine's RAM. The downside is that all scsi_debug devices share
>> the same ramdisk image. This option doubles the amount of ramdisk
>> storage space with the first, third, fifth (etc) created
>> scsi_debug devices using the first ramdisk image while the second,
>> fourth, sixth (etc) created scsi_debug devices using the second
>> ramdisk image.
>>
>> The reason for doing this is to check that (partial) disk to disk
>> copies based on scsi_debug devices have actually worked properly.
>> As an example: assume /dev/sdb and /dev/sg1 are the same
>> scsi_debug device, while /dev/sdc and /dev/sg2 are also the
>> same scsi_debug device. With doublestore=1 they will have
>> different ramdisk images. Then the following pseudocode could
>> be executed to check the if sgh_dd copy worked:
>>      dd if=/dev/urandom of=/dev/sdb
>>      sgh_dd if=/dev/sg1 of=/dev/sg2 [plus option(s) to test]
>>      cmp /dev/sdb /dev/sdc
>>
>> If the cmp fails then the copy has failed (or some other
>> mechanism wrote to /dev/sdb or /dev/sdc in the interim).
>>
>> Signed-off-by: Douglas Gilbert <[e-mail]>
>> ---
>>   drivers/scsi/scsi_debug.c | 264 +++++++++++++++++++++++++++-----------
>>   1 file changed, 186 insertions(+), 78 deletions(-)
>>
> Nice use-case, but really should be documented somewhere.
> Otherwise:

As stated in the cover letter:
     "If and when this patchset is accepted, this page will be
      updated:   http://sg.danny.cz/sg/sdebug26.html ".
Doing it before then can lead to confusion if the patchset
is not accepted.

One of my scripts for testing the sg driver uses that pattern
with the cmp replaced by:
     sgh_dd --verify if=/dev/sg1 of=/dev/sg2

The sg_dd utility also has a --verify option and I plan to add
one the the ddpt utility. The pseudo SCSI command sequence that
it performs is:
     PRE-FETCH(sg2, IMMED)
     READ(sg1)
     VERIFY(sg2, BYTCHK=1, data from READ)

Comparing this pattern to the standard READ-READ approach:
   - uses less host CPU and ram resources
   - possibly faster in the inequality case
   - faster if the host does the READ-READ sequentially

Doug Gilbert

Patch

diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index 18c07e9ace15..a32be83b22c7 100644
--- a/drivers/scsi/scsi_debug.c
+++ b/drivers/scsi/scsi_debug.c
@@ -108,6 +108,7 @@  static const char *sdebug_version_date = "20190125";
 #define DEF_DEV_SIZE_MB   8
 #define DEF_DIF 0
 #define DEF_DIX 0
+#define DEF_DOUBLESTORE false
 #define DEF_D_SENSE   0
 #define DEF_EVERY_NTH   0
 #define DEF_FAKE_RW	0
@@ -255,6 +256,7 @@  struct sdebug_dev_info {
 	unsigned long uas_bm[1];
 	atomic_t num_in_q;
 	atomic_t stopped;
+	int sdg_devnum;
 	bool used;
 };
 
@@ -633,6 +635,7 @@  static int sdebug_max_queue = SDEBUG_CANQUEUE;	/* per submit queue */
 static unsigned int sdebug_medium_error_start = OPT_MEDIUM_ERR_ADDR;
 static int sdebug_medium_error_count = OPT_MEDIUM_ERR_NUM;
 static atomic_t retired_max_queue;	/* if > 0 then was prior max_queue */
+static atomic_t a_sdg_devnum;
 static int sdebug_ndelay = DEF_NDELAY;	/* if > 0 then unit is nanoseconds */
 static int sdebug_no_lun_0 = DEF_NO_LUN_0;
 static int sdebug_no_uld;
@@ -658,6 +661,7 @@  static unsigned int sdebug_unmap_max_desc = DEF_UNMAP_MAX_DESC;
 static unsigned int sdebug_write_same_length = DEF_WRITESAME_LENGTH;
 static int sdebug_uuid_ctl = DEF_UUID_CTL;
 static bool sdebug_random = DEF_RANDOM;
+static bool sdebug_doublestore = DEF_DOUBLESTORE;
 static bool sdebug_removable = DEF_REMOVABLE;
 static bool sdebug_clustering;
 static bool sdebug_host_lock = DEF_HOST_LOCK;
@@ -681,7 +685,7 @@  static int sdebug_sectors_per;		/* sectors per cylinder */
 static LIST_HEAD(sdebug_host_list);
 static DEFINE_SPINLOCK(sdebug_host_list_lock);
 
-static unsigned char *fake_storep;	/* ramdisk storage */
+static u8 *fake_store_a[2];		/* ramdisk storage */
 static struct t10_pi_tuple *dif_storep;	/* protection info */
 static void *map_storep;		/* provisioning map */
 
@@ -699,6 +703,9 @@  static int submit_queues = DEF_SUBMIT_QUEUES;  /* > 1 for multi-queue (mq) */
 static struct sdebug_queue *sdebug_q_arr;  /* ptr to array of submit queues */
 
 static DEFINE_RWLOCK(atomic_rw);
+static DEFINE_RWLOCK(atomic_rw2);
+
+static rwlock_t *ramdisk_lck_a[2];
 
 static char sdebug_proc_name[] = MY_NAME;
 static const char *my_name = MY_NAME;
@@ -730,11 +737,11 @@  static inline bool scsi_debug_lbp(void)
 		(sdebug_lbpu || sdebug_lbpws || sdebug_lbpws10);
 }
 
-static void *lba2fake_store(unsigned long long lba)
+static void *lba2fake_store(unsigned long long lba, int acc_num)
 {
 	lba = do_div(lba, sdebug_store_sectors);
 
-	return fake_storep + lba * sdebug_sector_size;
+	return fake_store_a[acc_num % 2] + lba * sdebug_sector_size;
 }
 
 static struct t10_pi_tuple *dif_store(sector_t sector)
@@ -1043,7 +1050,7 @@  static int p_fill_from_dev_buffer(struct scsi_cmnd *scp, const void *arr,
 		 __func__, off_dst, scsi_bufflen(scp), act_len,
 		 scsi_get_resid(scp));
 	n = scsi_bufflen(scp) - (off_dst + act_len);
-	scsi_set_resid(scp, min(scsi_get_resid(scp), n));
+	scsi_set_resid(scp, min_t(int, scsi_get_resid(scp), n));
 	return 0;
 }
 
@@ -1535,7 +1542,7 @@  static int resp_inquiry(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 	}
 	put_unaligned_be16(0x2100, arr + n);	/* SPL-4 no version claimed */
 	ret = fill_from_dev_buffer(scp, arr,
-			    min(alloc_len, SDEBUG_LONG_INQ_SZ));
+			    min_t(int, alloc_len, SDEBUG_LONG_INQ_SZ));
 	kfree(arr);
 	return ret;
 }
@@ -1690,7 +1697,7 @@  static int resp_readcap16(struct scsi_cmnd *scp,
 	}
 
 	return fill_from_dev_buffer(scp, arr,
-				    min(alloc_len, SDEBUG_READCAP16_ARR_SZ));
+			    min_t(int, alloc_len, SDEBUG_READCAP16_ARR_SZ));
 }
 
 #define SDEBUG_MAX_TGTPGS_ARR_SZ 1412
@@ -1764,9 +1771,9 @@  static int resp_report_tgtpgs(struct scsi_cmnd *scp,
 	 * - The constructed command length
 	 * - The maximum array size
 	 */
-	rlen = min(alen,n);
+	rlen = min_t(int, alen, n);
 	ret = fill_from_dev_buffer(scp, arr,
-				   min(rlen, SDEBUG_MAX_TGTPGS_ARR_SZ));
+			   min_t(int, rlen, SDEBUG_MAX_TGTPGS_ARR_SZ));
 	kfree(arr);
 	return ret;
 }
@@ -2268,7 +2275,7 @@  static int resp_mode_sense(struct scsi_cmnd *scp,
 		arr[0] = offset - 1;
 	else
 		put_unaligned_be16((offset - 2), arr + 0);
-	return fill_from_dev_buffer(scp, arr, min(alloc_len, offset));
+	return fill_from_dev_buffer(scp, arr, min_t(int, alloc_len, offset));
 }
 
 #define SDEBUG_MAX_MSELECT_SZ 512
@@ -2453,9 +2460,9 @@  static int resp_log_sense(struct scsi_cmnd *scp,
 		mk_sense_invalid_fld(scp, SDEB_IN_CDB, 3, -1);
 		return check_condition_result;
 	}
-	len = min(get_unaligned_be16(arr + 2) + 4, alloc_len);
+	len = min_t(int, get_unaligned_be16(arr + 2) + 4, alloc_len);
 	return fill_from_dev_buffer(scp, arr,
-		    min(len, SDEBUG_MAX_INQ_ARR_SZ));
+		    min_t(int, len, SDEBUG_MAX_INQ_ARR_SZ));
 }
 
 static inline int check_device_access_params(struct scsi_cmnd *scp,
@@ -2478,6 +2485,18 @@  static inline int check_device_access_params(struct scsi_cmnd *scp,
 	return 0;
 }
 
+static int scp2acc_num(struct scsi_cmnd *scp)
+{
+	if (sdebug_doublestore) {
+		struct scsi_device *sdp = scp->device;
+		struct sdebug_dev_info *devip =
+				(struct sdebug_dev_info *)sdp->hostdata;
+
+		return devip->sdg_devnum;
+	}
+	return 0;
+}
+
 /* Returns number of bytes copied or -1 if error. */
 static int do_device_access(struct scsi_cmnd *scmd, u32 sg_skip, u64 lba,
 			    u32 num, bool do_write)
@@ -2486,6 +2505,7 @@  static int do_device_access(struct scsi_cmnd *scmd, u32 sg_skip, u64 lba,
 	u64 block, rest = 0;
 	struct scsi_data_buffer *sdb = &scmd->sdb;
 	enum dma_data_direction dir;
+	u8 *fsp;
 
 	if (do_write) {
 		dir = DMA_TO_DEVICE;
@@ -2498,20 +2518,21 @@  static int do_device_access(struct scsi_cmnd *scmd, u32 sg_skip, u64 lba,
 		return 0;
 	if (scmd->sc_data_direction != dir)
 		return -1;
+	fsp = fake_store_a[scp2acc_num(scmd) % 2];
 
 	block = do_div(lba, sdebug_store_sectors);
 	if (block + num > sdebug_store_sectors)
 		rest = block + num - sdebug_store_sectors;
 
 	ret = sg_copy_buffer(sdb->table.sgl, sdb->table.nents,
-		   fake_storep + (block * sdebug_sector_size),
+		   fsp + (block * sdebug_sector_size),
 		   (num - rest) * sdebug_sector_size, sg_skip, do_write);
 	if (ret != (num - rest) * sdebug_sector_size)
 		return ret;
 
 	if (rest) {
 		ret += sg_copy_buffer(sdb->table.sgl, sdb->table.nents,
-			    fake_storep, rest * sdebug_sector_size,
+			    fsp, rest * sdebug_sector_size,
 			    sg_skip + ((num - rest) * sdebug_sector_size),
 			    do_write);
 	}
@@ -2519,34 +2540,48 @@  static int do_device_access(struct scsi_cmnd *scmd, u32 sg_skip, u64 lba,
 	return ret;
 }
 
+/* Returns number of bytes copied or -1 if error. */
+static int do_dout_fetch(struct scsi_cmnd *scmd, u32 num, u8 *doutp)
+{
+	struct scsi_data_buffer *sdb = &scmd->sdb;
+
+	if (!sdb->length)
+		return 0;
+	if (scmd->sc_data_direction != DMA_TO_DEVICE)
+		return -1;
+	return sg_copy_buffer(sdb->table.sgl, sdb->table.nents, doutp,
+			      num * sdebug_sector_size, 0, true);
+}
+
 /* If lba2fake_store(lba,num) compares equal to arr(num), then copy top half of
  * arr into lba2fake_store(lba,num) and return true. If comparison fails then
  * return false. */
-static bool comp_write_worker(u64 lba, u32 num, const u8 *arr)
+static bool comp_write_worker(u64 lba, u32 num, const u8 *arr, int acc_num)
 {
 	bool res;
 	u64 block, rest = 0;
 	u32 store_blks = sdebug_store_sectors;
 	u32 lb_size = sdebug_sector_size;
+	u8 *fsp;
 
 	block = do_div(lba, store_blks);
 	if (block + num > store_blks)
 		rest = block + num - store_blks;
 
-	res = !memcmp(fake_storep + (block * lb_size), arr,
-		      (num - rest) * lb_size);
+	fsp = fake_store_a[acc_num % 2];
+
+	res = !memcmp(fsp + (block * lb_size), arr, (num - rest) * lb_size);
 	if (!res)
 		return res;
 	if (rest)
-		res = memcmp(fake_storep, arr + ((num - rest) * lb_size),
+		res = memcmp(fsp, arr + ((num - rest) * lb_size),
 			     rest * lb_size);
 	if (!res)
 		return res;
 	arr += num * lb_size;
-	memcpy(fake_storep + (block * lb_size), arr, (num - rest) * lb_size);
+	memcpy(fsp + (block * lb_size), arr, (num - rest) * lb_size);
 	if (rest)
-		memcpy(fake_storep, arr + ((num - rest) * lb_size),
-		       rest * lb_size);
+		memcpy(fsp, arr + ((num - rest) * lb_size), rest * lb_size);
 	return res;
 }
 
@@ -2605,7 +2640,7 @@  static void dif_copy_prot(struct scsi_cmnd *SCpnt, sector_t sector,
 			(read ? SG_MITER_TO_SG : SG_MITER_FROM_SG));
 
 	while (sg_miter_next(&miter) && resid > 0) {
-		size_t len = min(miter.length, resid);
+		size_t len = min_t(size_t, miter.length, resid);
 		void *start = dif_store(sector);
 		size_t rest = 0;
 
@@ -2632,12 +2667,12 @@  static void dif_copy_prot(struct scsi_cmnd *SCpnt, sector_t sector,
 	sg_miter_stop(&miter);
 }
 
-static int prot_verify_read(struct scsi_cmnd *SCpnt, sector_t start_sec,
+static int prot_verify_read(struct scsi_cmnd *scp, sector_t start_sec,
 			    unsigned int sectors, u32 ei_lba)
 {
 	unsigned int i;
-	struct t10_pi_tuple *sdt;
 	sector_t sector;
+	struct t10_pi_tuple *sdt;
 
 	for (i = 0; i < sectors; i++, ei_lba++) {
 		int ret;
@@ -2648,14 +2683,15 @@  static int prot_verify_read(struct scsi_cmnd *SCpnt, sector_t start_sec,
 		if (sdt->app_tag == cpu_to_be16(0xffff))
 			continue;
 
-		ret = dif_verify(sdt, lba2fake_store(sector), sector, ei_lba);
+		ret = dif_verify(sdt, lba2fake_store(sector, scp2acc_num(scp)),
+				 sector, ei_lba);
 		if (ret) {
 			dif_errors++;
 			return ret;
 		}
 	}
 
-	dif_copy_prot(SCpnt, start_sec, sectors, true);
+	dif_copy_prot(scp, start_sec, sectors, true);
 	dix_reads++;
 
 	return 0;
@@ -2665,10 +2701,11 @@  static int resp_read_dt0(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 {
 	u8 *cmd = scp->cmnd;
 	struct sdebug_queued_cmd *sqcp;
-	u64 lba;
 	u32 num;
 	u32 ei_lba;
+	int acc_num = scp2acc_num(scp);
 	unsigned long iflags;
+	u64 lba;
 	int ret;
 	bool check_prot;
 
@@ -2752,21 +2789,22 @@  static int resp_read_dt0(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 		return check_condition_result;
 	}
 
-	read_lock_irqsave(&atomic_rw, iflags);
+	read_lock_irqsave(ramdisk_lck_a[acc_num % 2], iflags);
 
 	/* DIX + T10 DIF */
 	if (unlikely(sdebug_dix && scsi_prot_sg_count(scp))) {
 		int prot_ret = prot_verify_read(scp, lba, num, ei_lba);
 
 		if (prot_ret) {
-			read_unlock_irqrestore(&atomic_rw, iflags);
+			read_unlock_irqrestore(ramdisk_lck_a[acc_num % 2],
+					       iflags);
 			mk_sense_buffer(scp, ABORTED_COMMAND, 0x10, prot_ret);
 			return illegal_condition_result;
 		}
 	}
 
 	ret = do_device_access(scp, 0, lba, num, false);
-	read_unlock_irqrestore(&atomic_rw, iflags);
+	read_unlock_irqrestore(ramdisk_lck_a[acc_num % 2], iflags);
 	if (unlikely(ret == -1))
 		return DID_ERROR << 16;
 
@@ -2938,9 +2976,10 @@  static void map_region(sector_t lba, unsigned int len)
 	}
 }
 
-static void unmap_region(sector_t lba, unsigned int len)
+static void unmap_region(sector_t lba, unsigned int len, int acc_num)
 {
 	sector_t end = lba + len;
+	u8 *fsp = fake_store_a[acc_num % 2];
 
 	while (lba < end) {
 		unsigned long index = lba_to_map_index(lba);
@@ -2950,8 +2989,7 @@  static void unmap_region(sector_t lba, unsigned int len)
 		    index < map_size) {
 			clear_bit(index, map_storep);
 			if (sdebug_lbprz) {  /* for LBPRZ=2 return 0xff_s */
-				memset(fake_storep +
-				       lba * sdebug_sector_size,
+				memset(fsp + lba * sdebug_sector_size,
 				       (sdebug_lbprz & 1) ? 0 : 0xff,
 				       sdebug_sector_size *
 				       sdebug_unmap_granularity);
@@ -2968,13 +3006,14 @@  static void unmap_region(sector_t lba, unsigned int len)
 
 static int resp_write_dt0(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 {
-	u8 *cmd = scp->cmnd;
-	u64 lba;
+	bool check_prot;
 	u32 num;
 	u32 ei_lba;
-	unsigned long iflags;
+	int acc_num = scp2acc_num(scp);
 	int ret;
-	bool check_prot;
+	unsigned long iflags;
+	u64 lba;
+	u8 *cmd = scp->cmnd;
 
 	switch (cmd[0]) {
 	case WRITE_16:
@@ -3030,14 +3069,15 @@  static int resp_write_dt0(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 	ret = check_device_access_params(scp, lba, num, true);
 	if (ret)
 		return ret;
-	write_lock_irqsave(&atomic_rw, iflags);
+	write_lock_irqsave(ramdisk_lck_a[acc_num % 2], iflags);
 
 	/* DIX + T10 DIF */
 	if (unlikely(sdebug_dix && scsi_prot_sg_count(scp))) {
 		int prot_ret = prot_verify_write(scp, lba, num, ei_lba);
 
 		if (prot_ret) {
-			write_unlock_irqrestore(&atomic_rw, iflags);
+			write_unlock_irqrestore(ramdisk_lck_a[acc_num % 2],
+						iflags);
 			mk_sense_buffer(scp, ILLEGAL_REQUEST, 0x10, prot_ret);
 			return illegal_condition_result;
 		}
@@ -3046,7 +3086,7 @@  static int resp_write_dt0(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 	ret = do_device_access(scp, 0, lba, num, true);
 	if (unlikely(scsi_debug_lbp()))
 		map_region(lba, num);
-	write_unlock_irqrestore(&atomic_rw, iflags);
+	write_unlock_irqrestore(ramdisk_lck_a[acc_num % 2], iflags);
 	if (unlikely(-1 == ret))
 		return DID_ERROR << 16;
 	else if (unlikely(sdebug_verbose &&
@@ -3092,6 +3132,7 @@  static int resp_write_scat(struct scsi_cmnd *scp,
 	u32 num, num_by, bt_len, lbdof_blen, sg_off, cum_lb;
 	u32 lb_size = sdebug_sector_size;
 	u32 ei_lba;
+	int acc_num = scp2acc_num(scp);
 	u64 lba;
 	unsigned long iflags;
 	int ret, res;
@@ -3155,7 +3196,7 @@  static int resp_write_scat(struct scsi_cmnd *scp,
 		goto err_out;
 	}
 
-	write_lock_irqsave(&atomic_rw, iflags);
+	write_lock_irqsave(ramdisk_lck_a[acc_num % 2], iflags);
 	sg_off = lbdof_blen;
 	/* Spec says Buffer xfer Length field in number of LBs in dout */
 	cum_lb = 0;
@@ -3238,7 +3279,7 @@  static int resp_write_scat(struct scsi_cmnd *scp,
 	}
 	ret = 0;
 err_out_unlock:
-	write_unlock_irqrestore(&atomic_rw, iflags);
+	write_unlock_irqrestore(ramdisk_lck_a[acc_num % 2], iflags);
 err_out:
 	kfree(lrdp);
 	return ret;
@@ -3247,27 +3288,30 @@  static int resp_write_scat(struct scsi_cmnd *scp,
 static int resp_write_same(struct scsi_cmnd *scp, u64 lba, u32 num,
 			   u32 ei_lba, bool unmap, bool ndob)
 {
-	int ret;
 	unsigned long iflags;
 	unsigned long long i;
-	u32 lb_size = sdebug_sector_size;
 	u64 block, lbaa;
+	u32 lb_size = sdebug_sector_size;
+	int ret;
+	int acc_num = scp2acc_num(scp);
 	u8 *fs1p;
+	u8 *fsp;
 
 	ret = check_device_access_params(scp, lba, num, true);
 	if (ret)
 		return ret;
 
-	write_lock_irqsave(&atomic_rw, iflags);
+	write_lock_irqsave(ramdisk_lck_a[acc_num % 2], iflags);
 
 	if (unmap && scsi_debug_lbp()) {
-		unmap_region(lba, num);
+		unmap_region(lba, num, acc_num);
 		goto out;
 	}
 	lbaa = lba;
 	block = do_div(lbaa, sdebug_store_sectors);
 	/* if ndob then zero 1 logical block, else fetch 1 logical block */
-	fs1p = fake_storep + (block * lb_size);
+	fsp = fake_store_a[acc_num % 2];
+	fs1p = fsp + (block * lb_size);
 	if (ndob) {
 		memset(fs1p, 0, lb_size);
 		ret = 0;
@@ -3275,7 +3319,7 @@  static int resp_write_same(struct scsi_cmnd *scp, u64 lba, u32 num,
 		ret = fetch_to_dev_buffer(scp, fs1p, lb_size);
 
 	if (-1 == ret) {
-		write_unlock_irqrestore(&atomic_rw, iflags);
+		write_unlock_irqrestore(ramdisk_lck_a[acc_num % 2], iflags);
 		return DID_ERROR << 16;
 	} else if (sdebug_verbose && !ndob && (ret < lb_size))
 		sdev_printk(KERN_INFO, scp->device,
@@ -3286,12 +3330,12 @@  static int resp_write_same(struct scsi_cmnd *scp, u64 lba, u32 num,
 	for (i = 1 ; i < num ; i++) {
 		lbaa = lba + i;
 		block = do_div(lbaa, sdebug_store_sectors);
-		memmove(fake_storep + (block * lb_size), fs1p, lb_size);
+		memmove(fsp + (block * lb_size), fs1p, lb_size);
 	}
 	if (scsi_debug_lbp())
 		map_region(lba, num);
 out:
-	write_unlock_irqrestore(&atomic_rw, iflags);
+	write_unlock_irqrestore(ramdisk_lck_a[acc_num % 2], iflags);
 
 	return 0;
 }
@@ -3403,7 +3447,6 @@  static int resp_comp_write(struct scsi_cmnd *scp,
 {
 	u8 *cmd = scp->cmnd;
 	u8 *arr;
-	u8 *fake_storep_hold;
 	u64 lba;
 	u32 dnum;
 	u32 lb_size = sdebug_sector_size;
@@ -3411,6 +3454,7 @@  static int resp_comp_write(struct scsi_cmnd *scp,
 	unsigned long iflags;
 	int ret;
 	int retval = 0;
+	int acc_num = scp2acc_num(scp);
 
 	lba = get_unaligned_be64(cmd + 2);
 	num = cmd[13];		/* 1 to a maximum of 255 logical blocks */
@@ -3437,14 +3481,9 @@  static int resp_comp_write(struct scsi_cmnd *scp,
 		return check_condition_result;
 	}
 
-	write_lock_irqsave(&atomic_rw, iflags);
+	write_lock_irqsave(ramdisk_lck_a[acc_num % 2], iflags);
 
-	/* trick do_device_access() to fetch both compare and write buffers
-	 * from data-in into arr. Safe (atomic) since write_lock held. */
-	fake_storep_hold = fake_storep;
-	fake_storep = arr;
-	ret = do_device_access(scp, 0, 0, dnum, true);
-	fake_storep = fake_storep_hold;
+	ret = do_dout_fetch(scp, dnum, arr);
 	if (ret == -1) {
 		retval = DID_ERROR << 16;
 		goto cleanup;
@@ -3452,7 +3491,7 @@  static int resp_comp_write(struct scsi_cmnd *scp,
 		sdev_printk(KERN_INFO, scp->device, "%s: compare_write: cdb "
 			    "indicated=%u, IO sent=%d bytes\n", my_name,
 			    dnum * lb_size, ret);
-	if (!comp_write_worker(lba, num, arr)) {
+	if (!comp_write_worker(lba, num, arr, scp2acc_num(scp))) {
 		mk_sense_buffer(scp, MISCOMPARE, MISCOMPARE_VERIFY_ASC, 0);
 		retval = check_condition_result;
 		goto cleanup;
@@ -3460,7 +3499,7 @@  static int resp_comp_write(struct scsi_cmnd *scp,
 	if (scsi_debug_lbp())
 		map_region(lba, num);
 cleanup:
-	write_unlock_irqrestore(&atomic_rw, iflags);
+	write_unlock_irqrestore(ramdisk_lck_a[acc_num % 2], iflags);
 	kfree(arr);
 	return retval;
 }
@@ -3477,6 +3516,7 @@  static int resp_unmap(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 	struct unmap_block_desc *desc;
 	unsigned int i, payload_len, descriptors;
 	int ret;
+	int acc_num = scp2acc_num(scp);
 	unsigned long iflags;
 
 
@@ -3505,7 +3545,7 @@  static int resp_unmap(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 
 	desc = (void *)&buf[8];
 
-	write_lock_irqsave(&atomic_rw, iflags);
+	write_lock_irqsave(ramdisk_lck_a[acc_num % 2], iflags);
 
 	for (i = 0 ; i < descriptors ; i++) {
 		unsigned long long lba = get_unaligned_be64(&desc[i].lba);
@@ -3515,13 +3555,13 @@  static int resp_unmap(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 		if (ret)
 			goto out;
 
-		unmap_region(lba, num);
+		unmap_region(lba, num, scp2acc_num(scp));
 	}
 
 	ret = 0;
 
 out:
-	write_unlock_irqrestore(&atomic_rw, iflags);
+	write_unlock_irqrestore(ramdisk_lck_a[acc_num % 2], iflags);
 	kfree(buf);
 
 	return ret;
@@ -3891,6 +3931,7 @@  static int scsi_debug_slave_configure(struct scsi_device *sdp)
 	if (sdebug_no_uld)
 		sdp->no_uld_attach = 1;
 	config_cdb_len(sdp);
+	devip->sdg_devnum = atomic_inc_return(&a_sdg_devnum);
 	return 0;
 }
 
@@ -4146,8 +4187,7 @@  static int scsi_debug_host_reset(struct scsi_cmnd *SCpnt)
 	return SUCCESS;
 }
 
-static void __init sdebug_build_parts(unsigned char *ramp,
-				      unsigned long store_size)
+static void sdebug_build_parts(unsigned char *ramp, unsigned long store_size)
 {
 	struct partition *pp;
 	int starts[SDEBUG_MAX_PARTS + 2];
@@ -4436,6 +4476,7 @@  module_param_named(delay, sdebug_jdelay, int, S_IRUGO | S_IWUSR);
 module_param_named(dev_size_mb, sdebug_dev_size_mb, int, S_IRUGO);
 module_param_named(dif, sdebug_dif, int, S_IRUGO);
 module_param_named(dix, sdebug_dix, int, S_IRUGO);
+module_param_named(doublestore, sdebug_doublestore, bool, S_IRUGO | S_IWUSR);
 module_param_named(dsense, sdebug_dsense, int, S_IRUGO | S_IWUSR);
 module_param_named(every_nth, sdebug_every_nth, int, S_IRUGO | S_IWUSR);
 module_param_named(fake_rw, sdebug_fake_rw, int, S_IRUGO | S_IWUSR);
@@ -4499,6 +4540,7 @@  MODULE_PARM_DESC(dev_size_mb, "size in MiB of ram shared by devs(def=8)");
 MODULE_PARM_DESC(dif, "data integrity field type: 0-3 (def=0)");
 MODULE_PARM_DESC(dix, "data integrity extensions mask (def=0)");
 MODULE_PARM_DESC(dsense, "use descriptor sense format(def=0 -> fixed)");
+MODULE_PARM_DESC(doublestore, "If set, 2 data buffers allocated, devices alternate between the two");
 MODULE_PARM_DESC(every_nth, "timeout every nth command(def=0)");
 MODULE_PARM_DESC(fake_rw, "fake reads/writes instead of copying (def=0)");
 MODULE_PARM_DESC(guard, "protection checksum: 0=crc, 1=ip (def=0)");
@@ -4788,16 +4830,24 @@  static ssize_t fake_rw_store(struct device_driver *ddp, const char *buf,
 		n = (n > 0);
 		sdebug_fake_rw = (sdebug_fake_rw > 0);
 		if (sdebug_fake_rw != n) {
-			if ((0 == n) && (NULL == fake_storep)) {
-				unsigned long sz =
-					(unsigned long)sdebug_dev_size_mb *
-					1048576;
-
-				fake_storep = vzalloc(sz);
-				if (NULL == fake_storep) {
-					pr_err("out of memory, 9\n");
+			unsigned long sz = (unsigned long)sdebug_dev_size_mb *
+					   1048576;
+
+			if (n == 0 && !fake_store_a[0]) {
+				fake_store_a[0] = vzalloc(sz);
+				if (!fake_store_a[0])
+					return -ENOMEM;
+				if (sdebug_num_parts > 0)
+					sdebug_build_parts(fake_store_a[0], sz);
+			}
+			if (sdebug_doublestore && n == 0 && !fake_store_a[1]) {
+				fake_store_a[1] = vzalloc(sz);
+				if (!fake_store_a[1]) {
+					sdebug_doublestore = false;
 					return -ENOMEM;
 				}
+				if (sdebug_num_parts > 0)
+					sdebug_build_parts(fake_store_a[1], sz);
 			}
 			sdebug_fake_rw = n;
 		}
@@ -4848,6 +4898,47 @@  static ssize_t dev_size_mb_show(struct device_driver *ddp, char *buf)
 }
 static DRIVER_ATTR_RO(dev_size_mb);
 
+static ssize_t doublestore_show(struct device_driver *ddp, char *buf)
+{
+	return scnprintf(buf, PAGE_SIZE, "%d\n", (int)sdebug_doublestore);
+}
+
+static ssize_t doublestore_store(struct device_driver *ddp, const char *buf,
+				 size_t count)
+{
+	int n;
+
+	if (count > 0 && kstrtoint(buf, 10, &n) == 0 && n >= 0) {
+		unsigned long iflags;
+
+		if (sdebug_doublestore == (n > 0))
+			return count;	/* no state change */
+		if (n <= 0) {
+			write_lock_irqsave(ramdisk_lck_a[1], iflags);
+			sdebug_doublestore = false;
+			vfree(fake_store_a[1]);
+			fake_store_a[1] = NULL;
+			write_unlock_irqrestore(ramdisk_lck_a[1], iflags);
+		} else {
+			unsigned long sz = (unsigned long)sdebug_dev_size_mb *
+					   1048576;
+			u8 *fsp = vzalloc(sz);
+
+			if (!fsp)
+				return -ENOMEM;
+			if (sdebug_num_parts > 0)
+				sdebug_build_parts(fsp, sz);
+			write_lock_irqsave(ramdisk_lck_a[1], iflags);
+			fake_store_a[1] = fsp;
+			sdebug_doublestore = true;
+			write_unlock_irqrestore(ramdisk_lck_a[1], iflags);
+		}
+		return count;
+	}
+	return -EINVAL;
+}
+static DRIVER_ATTR_RW(doublestore);
+
 static ssize_t num_parts_show(struct device_driver *ddp, char *buf)
 {
 	return scnprintf(buf, PAGE_SIZE, "%d\n", sdebug_num_parts);
@@ -5225,6 +5316,7 @@  static struct attribute *sdebug_drv_attrs[] = {
 	&driver_attr_opts.attr,
 	&driver_attr_ptype.attr,
 	&driver_attr_dsense.attr,
+	&driver_attr_doublestore.attr,
 	&driver_attr_fake_rw.attr,
 	&driver_attr_no_lun_0.attr,
 	&driver_attr_num_tgts.attr,
@@ -5266,7 +5358,10 @@  static int __init scsi_debug_init(void)
 	int k;
 	int ret;
 
+	ramdisk_lck_a[0] = &atomic_rw;
+	ramdisk_lck_a[1] = &atomic_rw2;
 	atomic_set(&retired_max_queue, 0);
+	atomic_set(&a_sdg_devnum, 0);
 
 	if (sdebug_ndelay >= 1000 * 1000 * 1000) {
 		pr_warn("ndelay must be less than 1 second, ignored\n");
@@ -5363,14 +5458,25 @@  static int __init scsi_debug_init(void)
 	}
 
 	if (sdebug_fake_rw == 0) {
-		fake_storep = vzalloc(sz);
-		if (NULL == fake_storep) {
-			pr_err("out of memory, 1\n");
+		fake_store_a[0] = vzalloc(sz);
+		if (!fake_store_a[0]) {
 			ret = -ENOMEM;
 			goto free_q_arr;
 		}
 		if (sdebug_num_parts > 0)
-			sdebug_build_parts(fake_storep, sz);
+			sdebug_build_parts(fake_store_a[0], sz);
+		if (sdebug_doublestore) {
+			fake_store_a[1] = vzalloc(sz);
+			if (!fake_store_a[1]) {
+				sdebug_doublestore = false;
+				vfree(fake_store_a[0]);
+				fake_store_a[0] = NULL;
+				ret = -ENOMEM;
+				goto free_q_arr;
+			}
+			if (sdebug_num_parts > 0)
+				sdebug_build_parts(fake_store_a[1], sz);
+		}
 	}
 
 	if (sdebug_dix) {
@@ -5467,7 +5573,8 @@  static int __init scsi_debug_init(void)
 free_vm:
 	vfree(map_storep);
 	vfree(dif_storep);
-	vfree(fake_storep);
+	vfree(fake_store_a[1]);
+	vfree(fake_store_a[0]);
 free_q_arr:
 	kfree(sdebug_q_arr);
 	return ret;
@@ -5487,7 +5594,8 @@  static void __exit scsi_debug_exit(void)
 
 	vfree(map_storep);
 	vfree(dif_storep);
-	vfree(fake_storep);
+	vfree(fake_store_a[1]);
+	vfree(fake_store_a[0]);
 	kfree(sdebug_q_arr);
 }