[PATCH] pNFS: Fix stripe mapping in block/scsi layout

[Sergey Bashirov <sergeybashirov@gmail.com>]

Because of integer division, we need to carefully calculate the
disk offset. Consider the example below for a stripe of 6 volumes,
a chunk size of 4096, and an offset of 70000.

chunk = div_u64(offset, dev->chunk_size) = 70000 / 4096 = 17
offset = chunk * dev->chunk_size = 17 * 4096 = 69632
disk_offset_wrong = div_u64(offset, dev->nr_children) = 69632 / 6 = 11605
disk_chunk = div_u64(chunk, dev->nr_children) = 17 / 6 = 2
disk_offset = disk_chunk * dev->chunk_size = 2 * 4096 = 8192

Signed-off-by: Sergey Bashirov <sergeybashirov@gmail.com>
---
 fs/nfs/blocklayout/dev.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/blocklayout/dev.c b/fs/nfs/blocklayout/dev.c
index cab8809f0e0f..44306ac22353 100644
--- a/fs/nfs/blocklayout/dev.c
+++ b/fs/nfs/blocklayout/dev.c
@@ -257,10 +257,11 @@ static bool bl_map_stripe(struct pnfs_block_dev *dev, u64 offset,
 	struct pnfs_block_dev *child;
 	u64 chunk;
 	u32 chunk_idx;
+	u64 disk_chunk;
 	u64 disk_offset;
 
 	chunk = div_u64(offset, dev->chunk_size);
-	div_u64_rem(chunk, dev->nr_children, &chunk_idx);
+	disk_chunk = div_u64_rem(chunk, dev->nr_children, &chunk_idx);
 
 	if (chunk_idx >= dev->nr_children) {
 		dprintk("%s: invalid chunk idx %d (%lld/%lld)\n",
@@ -273,7 +274,7 @@ static bool bl_map_stripe(struct pnfs_block_dev *dev, u64 offset,
 	offset = chunk * dev->chunk_size;
 
 	/* disk offset of the stripe */
-	disk_offset = div_u64(offset, dev->nr_children);
+	disk_offset = disk_chunk * dev->chunk_size;
 
 	child = &dev->children[chunk_idx];
 	child->map(child, disk_offset, map);
-- 
2.43.0