nvme-fc: Avoid preallocating big SGL for data

nvme_fc_create_io_queues() preallocates a big buffer for the IO SGL based
on SG_CHUNK_SIZE.

Modern DMA engines are often capable of dealing with very big segments so
the SG_CHUNK_SIZE is often too big. SG_CHUNK_SIZE results in a static 4KB
SGL allocation per command.

If a controller has lots of deep queues, preallocation for the sg list can
consume substantial amounts of memory. For nvme-fc, nr_hw_queues can be
128 and each queue's depth 128. This means the resulting preallocation
for the data SGL is 128*128*4K = 64MB per controller.

Switch to runtime allocation for SGL for lists longer than 2 entries. This
is the approach used by NVMe PCI so it should be reasonable for NVMeOF as
well. Runtime SGL allocation has always been the case for the legacy I/O
path so this is nothing new.
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>

drivers/nvme/host/fc.c

@@ -95,7 +95,7 @@ struct nvme_fc_fcp_op {
 
 struct nvme_fcp_op_w_sgl {
 	struct nvme_fc_fcp_op	op;
-	struct scatterlist	sgl[SG_CHUNK_SIZE];
+	struct scatterlist	sgl[NVME_INLINE_SG_CNT];
 	uint8_t			priv[0];
 };
 
@@ -2141,7 +2141,7 @@ nvme_fc_map_data(struct nvme_fc_ctrl *ctrl, struct request *rq,
 	freq->sg_table.sgl = freq->first_sgl;
 	ret = sg_alloc_table_chained(&freq->sg_table,
 			blk_rq_nr_phys_segments(rq), freq->sg_table.sgl,
-			SG_CHUNK_SIZE);
+			NVME_INLINE_SG_CNT);
 	if (ret)
 		return -ENOMEM;
 
@@ -2150,7 +2150,7 @@ nvme_fc_map_data(struct nvme_fc_ctrl *ctrl, struct request *rq,
 	freq->sg_cnt = fc_dma_map_sg(ctrl->lport->dev, freq->sg_table.sgl,
 				op->nents, rq_dma_dir(rq));
 	if (unlikely(freq->sg_cnt <= 0)) {
-		sg_free_table_chained(&freq->sg_table, SG_CHUNK_SIZE);
+		sg_free_table_chained(&freq->sg_table, NVME_INLINE_SG_CNT);
 		freq->sg_cnt = 0;
 		return -EFAULT;
 	}
@@ -2173,7 +2173,7 @@ nvme_fc_unmap_data(struct nvme_fc_ctrl *ctrl, struct request *rq,
 	fc_dma_unmap_sg(ctrl->lport->dev, freq->sg_table.sgl, op->nents,
 			rq_dma_dir(rq));
 
-	sg_free_table_chained(&freq->sg_table, SG_CHUNK_SIZE);
+	sg_free_table_chained(&freq->sg_table, NVME_INLINE_SG_CNT);
 
 	freq->sg_cnt = 0;
 }