pNFS: Fix stripe mapping in block/scsi layout

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>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250701122341.199112-1-sergeybashirov@gmail.com
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

diff --git a/fs/nfs/blocklayout/dev.c b/fs/nfs/blocklayout/dev.c
index cab8809f0e0f4864dcd4bfa6cdadaaba4421c1d2..44306ac22353bed51d521cb0b5308cf12b5c3bd5 100644 (file)
--- 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);