xen/blkfront: use blk_rq_map_sg to generate ring entries

On occasion, the request will apparently have more segments than we
fit into the ring. Jens says:

> The second problem is that the block layer then appears to create one
> too many segments, but from the dump it has rq->nr_phys_segments ==
> BLKIF_MAX_SEGMENTS_PER_REQUEST. I suspect the latter is due to
> xen-blkfront not handling the merging on its own. It should check that
> the new page doesn't form part of the previous page. The
> rq_for_each_segment() iterates all single bits in the request, not dma
> segments. The "easiest" way to do this is to call blk_rq_map_sg() and
> then iterate the mapped sg list. That will give you what you are
> looking for.

> Here's a test patch, compiles but otherwise untested. I spent more
> time figuring out how to enable XEN than to code it up, so YMMV!
> Probably the sg list wants to be put inside the ring and only
> initialized on allocation, then you can get rid of the sg on stack and
> sg_init_table() loop call in the function. I'll leave that, and the
> testing, to you.

[Moved sg array into info structure, and initialize once. -J]

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

1 files changed, 15 insertions, 15 deletions

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 918ef725de41..b6c8ce254359 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -40,6 +40,7 @@
 #include <linux/hdreg.h>
 #include <linux/cdrom.h>
 #include <linux/module.h>
+#include <linux/scatterlist.h>
 
 #include <xen/xenbus.h>
 #include <xen/grant_table.h>
@@ -82,6 +83,7 @@ struct blkfront_info
         enum blkif_state connected;
         int ring_ref;
         struct blkif_front_ring ring;
+        struct scatterlist sg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
         unsigned int evtchn, irq;
         struct request_queue *rq;
         struct work_struct work;
@@ -204,12 +206,11 @@ static int blkif_queue_request(struct request *req)
         struct blkfront_info *info = req->rq_disk->private_data;
         unsigned long buffer_mfn;
         struct blkif_request *ring_req;
-        struct req_iterator iter;
-        struct bio_vec *bvec;
         unsigned long id;
         unsigned int fsect, lsect;
-        int ref;
+        int i, ref;
         grant_ref_t gref_head;
+        struct scatterlist *sg;
 
         if (unlikely(info->connected != BLKIF_STATE_CONNECTED))
                 return 1;
@@ -238,12 +239,13 @@ static int blkif_queue_request(struct request *req)
         if (blk_barrier_rq(req))
                 ring_req->operation = BLKIF_OP_WRITE_BARRIER;
 
-        ring_req->nr_segments = 0;
-        rq_for_each_segment(bvec, req, iter) {
-                BUG_ON(ring_req->nr_segments == BLKIF_MAX_SEGMENTS_PER_REQUEST);
-                buffer_mfn = pfn_to_mfn(page_to_pfn(bvec->bv_page));
-                fsect = bvec->bv_offset >> 9;
-                lsect = fsect + (bvec->bv_len >> 9) - 1;
+        ring_req->nr_segments = blk_rq_map_sg(req->q, req, info->sg);
+        BUG_ON(ring_req->nr_segments > BLKIF_MAX_SEGMENTS_PER_REQUEST);
+
+        for_each_sg(info->sg, sg, ring_req->nr_segments, i) {
+                buffer_mfn = pfn_to_mfn(page_to_pfn(sg_page(sg)));
+                fsect = sg->offset >> 9;
+                lsect = fsect + (sg->length >> 9) - 1;
                 /* install a grant reference. */
                 ref = gnttab_claim_grant_reference(&gref_head);
                 BUG_ON(ref == -ENOSPC);
@@ -254,16 +256,12 @@ static int blkif_queue_request(struct request *req)
                                 buffer_mfn,
                                 rq_data_dir(req) );
 
-                info->shadow[id].frame[ring_req->nr_segments] =
-                                mfn_to_pfn(buffer_mfn);
-
-                ring_req->seg[ring_req->nr_segments] =
+                info->shadow[id].frame[i] = mfn_to_pfn(buffer_mfn);
+                ring_req->seg[i] =
                                 (struct blkif_request_segment) {
                                         .gref       = ref,
                                         .first_sect = fsect,
                                         .last_sect  = lsect };
-
-                ring_req->nr_segments++;
         }
 
         info->ring.req_prod_pvt++;
@@ -622,6 +620,8 @@ static int setup_blkring(struct xenbus_device *dev,
         SHARED_RING_INIT(sring);
         FRONT_RING_INIT(&info->ring, sring, PAGE_SIZE);
 
+        sg_init_table(info->sg, BLKIF_MAX_SEGMENTS_PER_REQUEST);
+
         err = xenbus_grant_ring(dev, virt_to_mfn(info->ring.sring));
         if (err < 0) {
                 free_page((unsigned long)sring);