Commit 81438498 authored by Sergey Bashirov's avatar Sergey Bashirov Committed by Trond Myklebust
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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: default avatarSergey Bashirov <sergeybashirov@gmail.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250701122341.199112-1-sergeybashirov@gmail.com


Signed-off-by: default avatarTrond Myklebust <trond.myklebust@hammerspace.com>
parent d897d816

fs/nfs/blocklayout/dev.c

+3 −2
Original line number Diff line number Diff line
@@ -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);