[RFC PATCH 2/3] tcm_iscsi: support multiple sizes in the scatterlist

[Paolo Bonzini]

The next patch will use multiple orders to satisfy the allocation
of the data buffers.

We need to support this in iscsi_map_iovec and iscsi_unmap_iovec.
The idea here is to walk each relevant page in the scatterlist
(which may not be every page in the scatterlist due to data_offset)
and kmap it.  iscsi_unmap_iovec uses the same algorithm, so that
each kunmap maps the kmap that was used in iscsi_map_iovec.

Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 drivers/target/iscsi/iscsi_target.c      |  106 ++++++++++++++++++++++++-----
 drivers/target/iscsi/iscsi_target_core.h |    2 +-
 2 files changed, 89 insertions(+), 19 deletions(-)

diff --git a/drivers/target/iscsi/iscsi_target.c b/drivers/target/iscsi/iscsi_target.c
index 224679e..dc4f5da 100644
--- a/drivers/target/iscsi/iscsi_target.c
+++ b/drivers/target/iscsi/iscsi_target.c
@@ -684,45 +684,115 @@ static int iscsit_map_iovec(
 	u32 data_offset,
 	u32 data_length)
 {
-	u32 i = 0;
 	struct scatterlist *sg;
-	unsigned int page_off;
+	u32 sg_off, sg_left, cur_len;
+	u32 sg_pfn = 0;
+	u32 i = 0;
 
-	/*
-	 * We know each entry in t_data_sg contains a page.
-	 */
-	sg = &cmd->se_cmd.t_data_sg[data_offset / PAGE_SIZE];
-	page_off = (data_offset % PAGE_SIZE);
+	sg = cmd->se_cmd.t_data_sg;
+	while (data_offset >= sg->length) {
+		data_offset -= sg->length;
+		sg++;
+	}
 
 	cmd->first_data_sg = sg;
-	cmd->first_data_sg_off = page_off;
+	cmd->first_data_sg_off = sg->offset + data_offset;
+	cmd->kmapped_length = data_length;
 
+	sg_left = 0;
 	while (data_length) {
-		u32 cur_len = min_t(u32, data_length, sg->length - page_off);
+		if (!sg_left) {
+			sg_off = sg->offset + data_offset;
+			sg_pfn = page_to_pfn(sg_page(sg));
+			sg_pfn += sg_off >> PAGE_SHIFT;
+			sg_off &= PAGE_SIZE - 1;
+
+			sg_left = min(sg->length, data_length);
+
+			/*
+			 * The next scatterlist is mapped from the beginning.
+			 */
+			sg++;
+			data_offset = 0;
+		}
 
-		iov[i].iov_base = kmap(sg_page(sg)) + sg->offset + page_off;
+		/*
+		 * Create an iovec for (a part of) this page.
+		 */
+		cur_len = min_t(u32, PAGE_SIZE - sg_off, sg_left);
+		BUG_ON(!cur_len);
+
+		iov[i].iov_base = kmap(pfn_to_page(sg_pfn)) + sg_off;
 		iov[i].iov_len = cur_len;
+		i++;
 
 		data_length -= cur_len;
-		page_off = 0;
-		sg = sg_next(sg);
-		i++;
-	}
+		sg_left -= cur_len;
 
-	cmd->kmapped_nents = i;
+		/*
+		 * The next pfn is mapped from the beginning.
+		 */
+		sg_pfn++;
+		sg_off = 0;
+	}
 
+	BUG_ON(sg_left);
 	return i;
 }
 
 static void iscsit_unmap_iovec(struct iscsi_cmd *cmd)
 {
-	u32 i;
 	struct scatterlist *sg;
+	u32 data_offset, data_length;
+	u32 sg_off, sg_left, cur_len;
+	u32 sg_pfn = 0;
+
+	if (!cmd->kmapped_length)
+		return;
 
 	sg = cmd->first_data_sg;
+	data_offset = cmd->first_data_sg_off - sg->offset;
+	data_length = cmd->kmapped_length;
+
+	/*
+	 * This loop must mirror exactly what is done in iscsi_map_iovec.
+	 */
+	sg_left = 0;
+	while (data_length) {
+		if (!sg_left) {
+			sg_off = sg->offset + data_offset;
+			sg_pfn = page_to_pfn(sg_page(sg));
+			sg_pfn += sg_off >> PAGE_SHIFT;
+			sg_off &= PAGE_SIZE - 1;
+
+			sg_left = min(sg->length, data_length);
+
+			/*
+			 * The next scatterlist is mapped from the beginning.
+			 */
+			sg++;
+			data_offset = 0;
+		}
+
+		/*
+		 * Create an iovec for (a part of) this page.
+		 */
+		cur_len = min_t(u32, PAGE_SIZE - sg_off, sg_left);
+		BUG_ON(!cur_len);
+
+		kunmap(pfn_to_page(sg_pfn));
+		data_length -= cur_len;
+		sg_left -= cur_len;
+
+		/*
+		 * The next pfn is mapped from the beginning.
+		 */
+		sg_pfn++;
+		sg_off = 0;
+	}
 
-	for (i = 0; i < cmd->kmapped_nents; i++)
-		kunmap(sg_page(&sg[i]));
+	BUG_ON(sg_left);
+	cmd->kmapped_length = 0;
 }
 
 static void iscsit_ack_from_expstatsn(struct iscsi_conn *conn, u32 exp_statsn)
diff --git a/drivers/target/iscsi/iscsi_target_core.h b/drivers/target/iscsi/iscsi_target_core.h
index 8a908b2..13bdaf1 100644
--- a/drivers/target/iscsi/iscsi_target_core.h
+++ b/drivers/target/iscsi/iscsi_target_core.h
@@ -472,7 +472,7 @@ struct iscsi_cmd {
 
 	struct scatterlist	*first_data_sg;
 	u32			first_data_sg_off;
-	u32			kmapped_nents;
+	u32			kmapped_length;
 
 }  ____cacheline_aligned;
 
-- 
1.7.1


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