SCSI: support for allocating large scatterlists

This is what enables large commands. If we need to allocate an
sgtable that doesn't fit in a single page, allocate several
SCSI_MAX_SG_SEGMENTS sized tables and chain them together.

SCSI defaults to large chained sg tables, if the arch supports it.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 162 insertions, 47 deletions

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 7e1dcc57f115..c75cb6ad6d94 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -36,33 +36,19 @@
 
 struct scsi_host_sg_pool {
         size_t          size;
-        char            *name; 
+        char            *name;
         struct kmem_cache       *slab;
         mempool_t       *pool;
 };
 
-#if (SCSI_MAX_PHYS_SEGMENTS < 32)
-#error SCSI_MAX_PHYS_SEGMENTS is too small
-#endif
-
-#define SP(x) { x, "sgpool-" #x } 
+#define SP(x) { x, "sgpool-" #x }
 static struct scsi_host_sg_pool scsi_sg_pools[] = {
         SP(8),
         SP(16),
         SP(32),
-#if (SCSI_MAX_PHYS_SEGMENTS > 32)
         SP(64),
-#if (SCSI_MAX_PHYS_SEGMENTS > 64)
         SP(128),
-#if (SCSI_MAX_PHYS_SEGMENTS > 128)
-        SP(256),
-#if (SCSI_MAX_PHYS_SEGMENTS > 256)
-#error SCSI_MAX_PHYS_SEGMENTS is too large
-#endif
-#endif
-#endif
-#endif
-};      
+};
 #undef SP
 
 static void scsi_run_queue(struct request_queue *q);
@@ -698,45 +684,126 @@ static struct scsi_cmnd *scsi_end_request(struct scsi_cmnd *cmd, int uptodate,
         return NULL;
 }
 
-struct scatterlist *scsi_alloc_sgtable(struct scsi_cmnd *cmd, gfp_t gfp_mask)
-{
-        struct scsi_host_sg_pool *sgp;
-        struct scatterlist *sgl;
+/*
+ * The maximum number of SG segments that we will put inside a scatterlist
+ * (unless chaining is used). Should ideally fit inside a single page, to
+ * avoid a higher order allocation.
+ */
+#define SCSI_MAX_SG_SEGMENTS    128
 
-        BUG_ON(!cmd->use_sg);
+/*
+ * Like SCSI_MAX_SG_SEGMENTS, but for archs that have sg chaining. This limit
+ * is totally arbitrary, a setting of 2048 will get you at least 8mb ios.
+ */
+#define SCSI_MAX_SG_CHAIN_SEGMENTS      2048
 
-        switch (cmd->use_sg) {
+static inline unsigned int scsi_sgtable_index(unsigned short nents)
+{
+        unsigned int index;
+
+        switch (nents) {
         case 1 ... 8:
-                cmd->sglist_len = 0;
+                index = 0;
                 break;
         case 9 ... 16:
-                cmd->sglist_len = 1;
+                index = 1;
                 break;
         case 17 ... 32:
-                cmd->sglist_len = 2;
+                index = 2;
                 break;
-#if (SCSI_MAX_PHYS_SEGMENTS > 32)
         case 33 ... 64:
-                cmd->sglist_len = 3;
+                index = 3;
                 break;
-#if (SCSI_MAX_PHYS_SEGMENTS > 64)
-        case 65 ... 128:
-                cmd->sglist_len = 4;
+        case 65 ... SCSI_MAX_SG_SEGMENTS:
+                index = 4;
                 break;
-#if (SCSI_MAX_PHYS_SEGMENTS  > 128)
-        case 129 ... 256:
-                cmd->sglist_len = 5;
-                break;
-#endif
-#endif
-#endif
         default:
-                return NULL;
+                printk(KERN_ERR "scsi: bad segment count=%d\n", nents);
+                BUG();
         }
 
-        sgp = scsi_sg_pools + cmd->sglist_len;
-        sgl = mempool_alloc(sgp->pool, gfp_mask);
-        return sgl;
+        return index;
+}
+
+struct scatterlist *scsi_alloc_sgtable(struct scsi_cmnd *cmd, gfp_t gfp_mask)
+{
+        struct scsi_host_sg_pool *sgp;
+        struct scatterlist *sgl, *prev, *ret;
+        unsigned int index;
+        int this, left;
+
+        BUG_ON(!cmd->use_sg);
+
+        left = cmd->use_sg;
+        ret = prev = NULL;
+        do {
+                this = left;
+                if (this > SCSI_MAX_SG_SEGMENTS) {
+                        this = SCSI_MAX_SG_SEGMENTS - 1;
+                        index = SG_MEMPOOL_NR - 1;
+                } else
+                        index = scsi_sgtable_index(this);
+
+                left -= this;
+
+                sgp = scsi_sg_pools + index;
+
+                sgl = mempool_alloc(sgp->pool, gfp_mask);
+                if (unlikely(!sgl))
+                        goto enomem;
+
+                memset(sgl, 0, sizeof(*sgl) * sgp->size);
+
+                /*
+                 * first loop through, set initial index and return value
+                 */
+                if (!ret) {
+                        cmd->sglist_len = index;
+                        ret = sgl;
+                }
+
+                /*
+                 * chain previous sglist, if any. we know the previous
+                 * sglist must be the biggest one, or we would not have
+                 * ended up doing another loop.
+                 */
+                if (prev)
+                        sg_chain(prev, SCSI_MAX_SG_SEGMENTS, sgl);
+
+                /*
+                 * don't allow subsequent mempool allocs to sleep, it would
+                 * violate the mempool principle.
+                 */
+                gfp_mask &= ~__GFP_WAIT;
+                gfp_mask |= __GFP_HIGH;
+                prev = sgl;
+        } while (left);
+
+        /*
+         * ->use_sg may get modified after dma mapping has potentially
+         * shrunk the number of segments, so keep a copy of it for free.
+         */
+        cmd->__use_sg = cmd->use_sg;
+        return ret;
+enomem:
+        if (ret) {
+                /*
+                 * Free entries chained off ret. Since we were trying to
+                 * allocate another sglist, we know that all entries are of
+                 * the max size.
+                 */
+                sgp = scsi_sg_pools + SG_MEMPOOL_NR - 1;
+                prev = ret;
+                ret = &ret[SCSI_MAX_SG_SEGMENTS - 1];
+
+                while ((sgl = sg_chain_ptr(ret)) != NULL) {
+                        ret = &sgl[SCSI_MAX_SG_SEGMENTS - 1];
+                        mempool_free(sgl, sgp->pool);
+                }
+
+                mempool_free(prev, sgp->pool);
+        }
+        return NULL;
 }
 
 EXPORT_SYMBOL(scsi_alloc_sgtable);
@@ -748,6 +815,42 @@ void scsi_free_sgtable(struct scsi_cmnd *cmd)
 
         BUG_ON(cmd->sglist_len >= SG_MEMPOOL_NR);
 
+        /*
+         * if this is the biggest size sglist, check if we have
+         * chained parts we need to free
+         */
+        if (cmd->__use_sg > SCSI_MAX_SG_SEGMENTS) {
+                unsigned short this, left;
+                struct scatterlist *next;
+                unsigned int index;
+
+                left = cmd->__use_sg - (SCSI_MAX_SG_SEGMENTS - 1);
+                next = sg_chain_ptr(&sgl[SCSI_MAX_SG_SEGMENTS - 1]);
+                while (left && next) {
+                        sgl = next;
+                        this = left;
+                        if (this > SCSI_MAX_SG_SEGMENTS) {
+                                this = SCSI_MAX_SG_SEGMENTS - 1;
+                                index = SG_MEMPOOL_NR - 1;
+                        } else
+                                index = scsi_sgtable_index(this);
+
+                        left -= this;
+
+                        sgp = scsi_sg_pools + index;
+
+                        if (left)
+                                next = sg_chain_ptr(&sgl[sgp->size - 1]);
+
+                        mempool_free(sgl, sgp->pool);
+                }
+
+                /*
+                 * Restore original, will be freed below
+                 */
+                sgl = cmd->request_buffer;
+        }
+
         sgp = scsi_sg_pools + cmd->sglist_len;
         mempool_free(sgl, sgp->pool);
 }
@@ -988,7 +1091,6 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 static int scsi_init_io(struct scsi_cmnd *cmd)
 {
         struct request     *req = cmd->request;
-        struct scatterlist *sgpnt;
         int                count;
 
         /*
@@ -1001,14 +1103,13 @@ static int scsi_init_io(struct scsi_cmnd *cmd)
         /*
          * If sg table allocation fails, requeue request later.
          */
-        sgpnt = scsi_alloc_sgtable(cmd, GFP_ATOMIC);
-        if (unlikely(!sgpnt)) {
+        cmd->request_buffer = scsi_alloc_sgtable(cmd, GFP_ATOMIC);
+        if (unlikely(!cmd->request_buffer)) {
                 scsi_unprep_request(req);
                 return BLKPREP_DEFER;
         }
 
         req->buffer = NULL;
-        cmd->request_buffer = (char *) sgpnt;
         if (blk_pc_request(req))
                 cmd->request_bufflen = req->data_len;
         else
@@ -1533,8 +1634,22 @@ struct request_queue *__scsi_alloc_queue(struct Scsi_Host *shost,
         if (!q)
                 return NULL;
 
+        /*
+         * this limit is imposed by hardware restrictions
+         */
         blk_queue_max_hw_segments(q, shost->sg_tablesize);
-        blk_queue_max_phys_segments(q, SCSI_MAX_PHYS_SEGMENTS);
+
+        /*
+         * In the future, sg chaining support will be mandatory and this
+         * ifdef can then go away. Right now we don't have all archs
+         * converted, so better keep it safe.
+         */
+#ifdef ARCH_HAS_SG_CHAIN
+        blk_queue_max_phys_segments(q, SCSI_MAX_SG_CHAIN_SEGMENTS);
+#else
+        blk_queue_max_phys_segments(q, SCSI_MAX_SG_SEGMENTS);
+#endif
+
         blk_queue_max_sectors(q, shost->max_sectors);
         blk_queue_bounce_limit(q, scsi_calculate_bounce_limit(shost));
         blk_queue_segment_boundary(q, shost->dma_boundary);