scsi: lib/sg_pool.c: improve APIs for allocating sg pool

sg_alloc_table_chained() currently allows the caller to provide one
preallocated SGL and returns if the requested number isn't bigger than
size of that SGL. This is used to inline an SGL for an IO request.

However, scattergather code only allows that size of the 1st preallocated
SGL to be SG_CHUNK_SIZE(128). This means a substantial amount of memory
(4KB) is claimed for the SGL for each IO request. If the I/O is small, it
would be prudent to allocate a smaller SGL.

Introduce an extra parameter to sg_alloc_table_chained() and
sg_free_table_chained() for specifying size of the preallocated SGL.

Both __sg_free_table() and __sg_alloc_table() assume that each SGL has the
same size except for the last one.  Change the code to allow both functions
to accept a variable size for the 1st preallocated SGL.

[mkp: attempted to clarify commit desc]

Cc: Christoph Hellwig <hch@lst.de>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Ewan D. Milne <emilne@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: netdev@vger.kernel.org
Cc: linux-nvme@lists.infradead.org
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

1 files changed, 27 insertions, 10 deletions

diff --git a/lib/sg_pool.c b/lib/sg_pool.c
index d1c1e6388eaa..b3b8cf62ff49 100644
--- a/lib/sg_pool.c
+++ b/lib/sg_pool.c
@@ -69,18 +69,27 @@ static struct scatterlist *sg_pool_alloc(unsigned int nents, gfp_t gfp_mask)
 /**
  * sg_free_table_chained - Free a previously mapped sg table
  * @table:	The sg table header to use
- * @first_chunk: was first_chunk not NULL in sg_alloc_table_chained?
+ * @nents_first_chunk: size of the first_chunk SGL passed to
+ *		sg_alloc_table_chained
  *
  *  Description:
  *    Free an sg table previously allocated and setup with
  *    sg_alloc_table_chained().
  *
+ *    @nents_first_chunk has to be same with that same parameter passed
+ *    to sg_alloc_table_chained().
+ *
  **/
-void sg_free_table_chained(struct sg_table *table, bool first_chunk)
+void sg_free_table_chained(struct sg_table *table,
+		unsigned nents_first_chunk)
 {
-	if (first_chunk && table->orig_nents <= SG_CHUNK_SIZE)
+	if (table->orig_nents <= nents_first_chunk)
 		return;
-	__sg_free_table(table, SG_CHUNK_SIZE, first_chunk, sg_pool_free);
+
+	if (nents_first_chunk == 1)
+		nents_first_chunk = 0;
+
+	__sg_free_table(table, SG_CHUNK_SIZE, nents_first_chunk, sg_pool_free);
 }
 EXPORT_SYMBOL_GPL(sg_free_table_chained);
 
@@ -89,31 +98,39 @@ EXPORT_SYMBOL_GPL(sg_free_table_chained);
  * @table:	The sg table header to use
  * @nents:	Number of entries in sg list
  * @first_chunk: first SGL
+ * @nents_first_chunk: number of the SGL of @first_chunk
  *
  *  Description:
  *    Allocate and chain SGLs in an sg table. If @nents@ is larger than
- *    SG_CHUNK_SIZE a chained sg table will be setup.
+ *    @nents_first_chunk a chained sg table will be setup.
  *
  **/
 int sg_alloc_table_chained(struct sg_table *table, int nents,
-		struct scatterlist *first_chunk)
+		struct scatterlist *first_chunk, unsigned nents_first_chunk)
 {
 	int ret;
 
 	BUG_ON(!nents);
 
-	if (first_chunk) {
-		if (nents <= SG_CHUNK_SIZE) {
+	if (first_chunk && nents_first_chunk) {
+		if (nents <= nents_first_chunk) {
 			table->nents = table->orig_nents = nents;
 			sg_init_table(table->sgl, nents);
 			return 0;
 		}
 	}
 
+	/* User supposes that the 1st SGL includes real entry */
+	if (nents_first_chunk == 1) {
+		first_chunk = NULL;
+		nents_first_chunk = 0;
+	}
+
 	ret = __sg_alloc_table(table, nents, SG_CHUNK_SIZE,
-			       first_chunk, GFP_ATOMIC, sg_pool_alloc);
+			       first_chunk, nents_first_chunk,
+			       GFP_ATOMIC, sg_pool_alloc);
 	if (unlikely(ret))
-		sg_free_table_chained(table, (bool)first_chunk);
+		sg_free_table_chained(table, nents_first_chunk);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(sg_alloc_table_chained);