scatterlist: allow chaining to preallocated chunks

Blk-mq drivers usually preallocate their S/G list as part of the request,
but if we want to support the very large S/G lists currently supported by
the SCSI code that would tie up a lot of memory in the preallocated request
pool.  Add support to the scatterlist code so that it can initialize a
S/G list that uses a preallocated first chunks and dynamically allocated
additional chunks.  That way the scsi-mq code can preallocate a first
page worth of S/G entries as part of the request, and dynamically extend
the S/G list when needed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Webb Scales <webbnh@hp.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Tested-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Robert Elliott <elliott@hp.com>

1 files changed, 17 insertions, 8 deletions

diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 3a8e8e8fb2a5..b4415fceb7e7 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -165,6 +165,7 @@ static void sg_kfree(struct scatterlist *sg, unsigned int nents)
  * __sg_free_table - Free a previously mapped sg table
  * @table:      The sg table header to use
  * @max_ents:   The maximum number of entries per single scatterlist
+ * @skip_first_chunk: don't free the (preallocated) first scatterlist chunk
  * @free_fn:    Free function
  *
  *  Description:
@@ -174,7 +175,7 @@ static void sg_kfree(struct scatterlist *sg, unsigned int nents)
  *
  **/
 void __sg_free_table(struct sg_table *table, unsigned int max_ents,
-                     sg_free_fn *free_fn)
+                     bool skip_first_chunk, sg_free_fn *free_fn)
 {
         struct scatterlist *sgl, *next;
 
@@ -202,7 +203,10 @@ void __sg_free_table(struct sg_table *table, unsigned int max_ents,
                 }
 
                 table->orig_nents -= sg_size;
-                free_fn(sgl, alloc_size);
+                if (!skip_first_chunk) {
+                        free_fn(sgl, alloc_size);
+                        skip_first_chunk = false;
+                }
                 sgl = next;
         }
 
@@ -217,7 +221,7 @@ EXPORT_SYMBOL(__sg_free_table);
  **/
 void sg_free_table(struct sg_table *table)
 {
-        __sg_free_table(table, SG_MAX_SINGLE_ALLOC, sg_kfree);
+        __sg_free_table(table, SG_MAX_SINGLE_ALLOC, false, sg_kfree);
 }
 EXPORT_SYMBOL(sg_free_table);
 
@@ -241,8 +245,8 @@ EXPORT_SYMBOL(sg_free_table);
  *
  **/
 int __sg_alloc_table(struct sg_table *table, unsigned int nents,
-                     unsigned int max_ents, gfp_t gfp_mask,
-                     sg_alloc_fn *alloc_fn)
+                     unsigned int max_ents, struct scatterlist *first_chunk,
+                     gfp_t gfp_mask, sg_alloc_fn *alloc_fn)
 {
         struct scatterlist *sg, *prv;
         unsigned int left;
@@ -269,7 +273,12 @@ int __sg_alloc_table(struct sg_table *table, unsigned int nents,
 
                 left -= sg_size;
 
-                sg = alloc_fn(alloc_size, gfp_mask);
+                if (first_chunk) {
+                        sg = first_chunk;
+                        first_chunk = NULL;
+                } else {
+                        sg = alloc_fn(alloc_size, gfp_mask);
+                }
                 if (unlikely(!sg)) {
                         /*
                          * Adjust entry count to reflect that the last
@@ -324,9 +333,9 @@ int sg_alloc_table(struct sg_table *table, unsigned int nents, gfp_t gfp_mask)
         int ret;
 
         ret = __sg_alloc_table(table, nents, SG_MAX_SINGLE_ALLOC,
-                               gfp_mask, sg_kmalloc);
+                               NULL, gfp_mask, sg_kmalloc);
         if (unlikely(ret))
-                __sg_free_table(table, SG_MAX_SINGLE_ALLOC, sg_kfree);
+                __sg_free_table(table, SG_MAX_SINGLE_ALLOC, false, sg_kfree);
 
         return ret;
 }