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pNFS: Fix stripe mapping in block/scsi layout

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[ Upstream commit 81438498a285759f31e843ac4800f82a5ce6521f ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Sergey Bashirov 2025-07-01 15:21:48 +03:00 • committed by Greg Kroah-Hartman
commit bd2810033c
1 changed files with 3 additions and 2 deletions
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5
fs/nfs/blocklayout/dev.c
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@ -199,10 +199,11 @@ 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",
@ -215,7 +216,7 @@ 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);