Fix calculation of number of pages in a PRP List

The existing calculation underestimated the number of pages required
as it did not take into account the pointer at the end of each page.
The replacement calculation may overestimate the number of pages required
if the last page in the PRP List is entirely full.  By using ->npages
as a counter as we fill in the pages, we ensure that we don't try to
free a page that was never allocated.
Signed-off-by: Nisheeth Bhat <nisheeth.bhat@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>

drivers/block/nvme.c

@@ -265,7 +265,7 @@ static int nvme_submit_cmd(struct nvme_queue *nvmeq, struct nvme_command *cmd)
 }
 
 struct nvme_prps {
-	int npages;
+	int npages;		/* 0 means small pool in use */
 	dma_addr_t first_dma;
 	__le64 *list[0];
 };
@@ -347,7 +347,7 @@ static struct nvme_prps *nvme_setup_prps(struct nvme_dev *dev,
 	int offset = offset_in_page(dma_addr);
 	__le64 *prp_list;
 	dma_addr_t prp_dma;
-	int nprps, npages, i, prp_page;
+	int nprps, npages, i;
 	struct nvme_prps *prps = NULL;
 
 	cmd->prp1 = cpu_to_le64(dma_addr);
@@ -370,20 +370,20 @@ static struct nvme_prps *nvme_setup_prps(struct nvme_dev *dev,
 	}
 
 	nprps = DIV_ROUND_UP(length, PAGE_SIZE);
-	npages = DIV_ROUND_UP(8 * nprps, PAGE_SIZE);
+	npages = DIV_ROUND_UP(8 * nprps, PAGE_SIZE - 8);
 	prps = kmalloc(sizeof(*prps) + sizeof(__le64 *) * npages, gfp);
 	if (!prps) {
 		cmd->prp2 = cpu_to_le64(dma_addr);
 		*len = (*len - length) + PAGE_SIZE;
 		return prps;
 	}
-	prp_page = 0;
+
 	if (nprps <= (256 / 8)) {
 		pool = dev->prp_small_pool;
 		prps->npages = 0;
 	} else {
 		pool = dev->prp_page_pool;
-		prps->npages = npages;
+		prps->npages = 1;
 	}
 
 	prp_list = dma_pool_alloc(pool, gfp, &prp_dma);
@@ -393,7 +393,7 @@ static struct nvme_prps *nvme_setup_prps(struct nvme_dev *dev,
 		kfree(prps);
 		return NULL;
 	}
-	prps->list[prp_page++] = prp_list;
+	prps->list[0] = prp_list;
 	prps->first_dma = prp_dma;
 	cmd->prp2 = cpu_to_le64(prp_dma);
 	i = 0;
@@ -405,7 +405,7 @@ static struct nvme_prps *nvme_setup_prps(struct nvme_dev *dev,
 				*len = (*len - length);
 				return prps;
 			}
-			prps->list[prp_page++] = prp_list;
+			prps->list[prps->npages++] = prp_list;
 			prp_list[0] = old_prp_list[i - 1];
 			old_prp_list[i - 1] = cpu_to_le64(prp_dma);
 			i = 1;