From 50bc2f07546738dc233c8674d5ba5f3560a2d601 Mon Sep 17 00:00:00 2001
From: Sasha Levin <sashal@kernel.org>
Date: Tue, 1 Jul 2025 15:21:48 +0300
Subject: pNFS: Fix stripe mapping in block/scsi layout

From: Sergey Bashirov <sergeybashirov@gmail.com>

[ 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>
---
 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 65cbb5607a5f..61ee0b6c0fba 100644
--- a/fs/nfs/blocklayout/dev.c
+++ b/fs/nfs/blocklayout/dev.c
@@ -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);
-- 
2.39.5