scsi: target: tcmu: scatter_/gather_data_area() rework

scatter_data_area() and gather_data_area() are not easy to understand since
data is copied in nested loops over sg_list and tcmu dbi list. Since sg
list can contain only partly filled pages, the loop has to be prepared to
handle sg pages not matching dbi pages one by one.

Existing implementation uses kmap_atomic()/kunmap_atomic() due to
performance reasons. But instead of using these calls strictly nested for
sg and dpi pages, the code holds the mappings in an overlapping way, which
indeed is a bug that would trigger on archs using highmem.

The scatterlist lib contains the sg_miter_start/_next/_stop functions which
can be used to simplify such complicated loops.

The new code now processes the dbi list in the outer loop, while sg list is
handled by the inner one. That way the code can take advantage of the
sg_miter_* family calls.

Calling sg_miter_stop() after the end of the inner loop enforces strict
nesting of atomic kmaps.

Since the nested loops in scatter_/gather_data_area were very similar, I
replaced them by the new helper function tcmu_copy_data().

Link: https://lore.kernel.org/r/20201019115118.11949-1-bostroesser@gmail.comAcked-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/target_core_user.c

@@ -586,14 +586,15 @@ static inline void tcmu_cmd_set_block_cnts(struct tcmu_cmd *cmd)
 }
 
 static int new_block_to_iov(struct tcmu_dev *udev, struct tcmu_cmd *cmd,
-			    struct iovec **iov, int prev_dbi, int *remain)
+			    struct iovec **iov, int prev_dbi, int len)
 {
 	/* Get the next dbi */
 	int dbi = tcmu_cmd_get_dbi(cmd);
+
 	/* Do not add more than DATA_BLOCK_SIZE to iov */
-	int len = min_t(int, DATA_BLOCK_SIZE, *remain);
+	if (len > DATA_BLOCK_SIZE)
+		len = DATA_BLOCK_SIZE;
 
-	*remain -= len;
 	/*
 	 * The following code will gather and map the blocks to the same iovec
 	 * when the blocks are all next to each other.
@@ -618,8 +619,8 @@ static void tcmu_setup_iovs(struct tcmu_dev *udev, struct tcmu_cmd *cmd,
 	int dbi = -2;
 
 	/* We prepare the IOVs for DMA_FROM_DEVICE transfer direction */
-	while (data_length > 0)
-		dbi = new_block_to_iov(udev, cmd, iov, dbi, &data_length);
+	for (; data_length > 0; data_length -= DATA_BLOCK_SIZE)
+		dbi = new_block_to_iov(udev, cmd, iov, dbi, data_length);
 }
 
 static struct tcmu_cmd *tcmu_alloc_cmd(struct se_cmd *se_cmd)
@@ -688,67 +689,83 @@ static inline size_t head_to_end(size_t head, size_t size)
 
 #define UPDATE_HEAD(head, used, size) smp_store_release(&head, ((head % size) + used) % size)
 
-static void scatter_data_area(struct tcmu_dev *udev, struct tcmu_cmd *tcmu_cmd,
-			      struct iovec **iov)
+#define TCMU_SG_TO_DATA_AREA 1
+#define TCMU_DATA_AREA_TO_SG 2
+
+static inline void tcmu_copy_data(struct tcmu_dev *udev,
+				  struct tcmu_cmd *tcmu_cmd, uint32_t direction,
+				  struct scatterlist *sg, unsigned int sg_nents,
+				  struct iovec **iov, size_t data_len)
 {
-	struct se_cmd *se_cmd = tcmu_cmd->se_cmd;
 	/* start value of dbi + 1 must not be a valid dbi */
-	int i, dbi = -2;
-	int block_remaining = 0;
-	int data_len = se_cmd->data_length;
-	void *from, *to = NULL;
-	size_t copy_bytes, offset;
-	struct scatterlist *sg;
-	struct page *page = NULL;
-
-	for_each_sg(se_cmd->t_data_sg, sg, se_cmd->t_data_nents, i) {
-		int sg_remaining = sg->length;
-		from = kmap_atomic(sg_page(sg)) + sg->offset;
-		while (sg_remaining > 0) {
-			if (block_remaining == 0) {
-				if (to) {
-					flush_dcache_page(page);
-					kunmap_atomic(to);
-				}
-
-				/* get next dbi and add to IOVs */
-				dbi = new_block_to_iov(udev, tcmu_cmd, iov, dbi,
-						       &data_len);
-				page = tcmu_get_block_page(udev, dbi);
-				to = kmap_atomic(page);
-				block_remaining = DATA_BLOCK_SIZE;
-			}
+	int dbi = -2;
+	size_t block_remaining, cp_len;
+	struct sg_mapping_iter sg_iter;
+	unsigned int sg_flags;
+	struct page *page;
+	void *data_page_start, *data_addr;
 
-			copy_bytes = min_t(size_t, sg_remaining,
-					block_remaining);
-			offset = DATA_BLOCK_SIZE - block_remaining;
-			memcpy(to + offset, from + sg->length - sg_remaining,
-			       copy_bytes);
+	if (direction == TCMU_SG_TO_DATA_AREA)
+		sg_flags = SG_MITER_ATOMIC | SG_MITER_FROM_SG;
+	else
+		sg_flags = SG_MITER_ATOMIC | SG_MITER_TO_SG;
+	sg_miter_start(&sg_iter, sg, sg_nents, sg_flags);
 
-			sg_remaining -= copy_bytes;
-			block_remaining -= copy_bytes;
+	while (data_len) {
+		if (direction == TCMU_SG_TO_DATA_AREA)
+			dbi = new_block_to_iov(udev, tcmu_cmd, iov, dbi,
+					       data_len);
+		else
+			dbi = tcmu_cmd_get_dbi(tcmu_cmd);
+		page = tcmu_get_block_page(udev, dbi);
+		if (direction == TCMU_DATA_AREA_TO_SG)
+			flush_dcache_page(page);
+		data_page_start = kmap_atomic(page);
+		block_remaining = DATA_BLOCK_SIZE;
+
+		while (block_remaining && data_len) {
+			if (!sg_miter_next(&sg_iter)) {
+				/* set length to 0 to abort outer loop */
+				data_len = 0;
+				pr_debug("tcmu_move_data: aborting data copy due to exhausted sg_list\n");
+				break;
+			}
+			cp_len = min3(sg_iter.length, block_remaining, data_len);
+
+			data_addr = data_page_start +
+				    DATA_BLOCK_SIZE - block_remaining;
+			if (direction == TCMU_SG_TO_DATA_AREA)
+				memcpy(data_addr, sg_iter.addr, cp_len);
+			else
+				memcpy(sg_iter.addr, data_addr, cp_len);
+
+			data_len -= cp_len;
+			block_remaining -= cp_len;
+			sg_iter.consumed = cp_len;
 		}
-		kunmap_atomic(from - sg->offset);
-	}
+		sg_miter_stop(&sg_iter);
 
-	if (to) {
-		flush_dcache_page(page);
-		kunmap_atomic(to);
+		kunmap_atomic(data_page_start);
+		if (direction == TCMU_SG_TO_DATA_AREA)
+			flush_dcache_page(page);
 	}
 }
 
-static void gather_data_area(struct tcmu_dev *udev, struct tcmu_cmd *cmd,
+static void scatter_data_area(struct tcmu_dev *udev, struct tcmu_cmd *tcmu_cmd,
+			      struct iovec **iov)
+{
+	struct se_cmd *se_cmd = tcmu_cmd->se_cmd;
+
+	tcmu_copy_data(udev, tcmu_cmd, TCMU_SG_TO_DATA_AREA, se_cmd->t_data_sg,
+		       se_cmd->t_data_nents, iov, se_cmd->data_length);
+}
+
+static void gather_data_area(struct tcmu_dev *udev, struct tcmu_cmd *tcmu_cmd,
 			     bool bidi, uint32_t read_len)
 {
-	struct se_cmd *se_cmd = cmd->se_cmd;
-	int i, dbi;
-	int block_remaining = 0;
-	void *from = NULL, *to;
-	size_t copy_bytes, offset;
-	struct scatterlist *sg, *data_sg;
-	struct page *page;
+	struct se_cmd *se_cmd = tcmu_cmd->se_cmd;
+	struct scatterlist *data_sg;
 	unsigned int data_nents;
-	uint32_t count = 0;
 
 	if (!bidi) {
 		data_sg = se_cmd->t_data_sg;
@@ -759,46 +776,15 @@ static void gather_data_area(struct tcmu_dev *udev, struct tcmu_cmd *cmd,
 		 * buffer blocks, and before gathering the Data-In buffer
 		 * the Data-Out buffer blocks should be skipped.
 		 */
-		count = cmd->dbi_cnt - cmd->dbi_bidi_cnt;
+		tcmu_cmd_set_dbi_cur(tcmu_cmd,
+				     tcmu_cmd->dbi_cnt - tcmu_cmd->dbi_bidi_cnt);
 
 		data_sg = se_cmd->t_bidi_data_sg;
 		data_nents = se_cmd->t_bidi_data_nents;
 	}
 
-	tcmu_cmd_set_dbi_cur(cmd, count);
-
-	for_each_sg(data_sg, sg, data_nents, i) {
-		int sg_remaining = sg->length;
-		to = kmap_atomic(sg_page(sg)) + sg->offset;
-		while (sg_remaining > 0 && read_len > 0) {
-			if (block_remaining == 0) {
-				if (from)
-					kunmap_atomic(from);
-
-				block_remaining = DATA_BLOCK_SIZE;
-				dbi = tcmu_cmd_get_dbi(cmd);
-				page = tcmu_get_block_page(udev, dbi);
-				from = kmap_atomic(page);
-				flush_dcache_page(page);
-			}
-			copy_bytes = min_t(size_t, sg_remaining,
-					block_remaining);
-			if (read_len < copy_bytes)
-				copy_bytes = read_len;
-			offset = DATA_BLOCK_SIZE - block_remaining;
-			memcpy(to + sg->length - sg_remaining, from + offset,
-					copy_bytes);
-
-			sg_remaining -= copy_bytes;
-			block_remaining -= copy_bytes;
-			read_len -= copy_bytes;
-		}
-		kunmap_atomic(to - sg->offset);
-		if (read_len == 0)
-			break;
-	}
-	if (from)
-		kunmap_atomic(from);
+	tcmu_copy_data(udev, tcmu_cmd, TCMU_DATA_AREA_TO_SG, data_sg,
+		       data_nents, NULL, read_len);
 }
 
 static inline size_t spc_bitmap_free(unsigned long *bitmap, uint32_t thresh)