[PATCH 6.12 007/567] iomap: account for unaligned end offsets when truncating read range

Greg Kroah-Hartman
2026-01-06T17:16:12Z

From: Joanne Koong <joannelkoong@gmail.com>

[ Upstream commit 9d875e0eef8ec15b6b1da0cb9a0f8ed13efee89e ]

The end position to start truncating from may be at an offset into a
block, which under the current logic would result in overtruncation.

Adjust the calculation to account for unaligned end offsets.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://patch.msgid.link/20251111193658.3495942-3-joannelkoong@gmail.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/iomap/buffered-io.c | 22 ++++++++++++++++++++--
1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 258ac7bf658fd..397c96c25c31f 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -227,6 +227,22 @@ static void ifs_free(struct folio *folio)
kfree(ifs);
}

+/*
+ * Calculate how many bytes to truncate based off the number of blocks to
+ * truncate and the end position to start truncating from.
+ */
+static size_t iomap_bytes_to_truncate(loff_t end_pos, unsigned block_bits,
+ unsigned blocks_truncated)
+{
+ unsigned block_size = 1 << block_bits;
+ unsigned block_offset = end_pos & (block_size - 1);
+
+ if (!block_offset)
+ return blocks_truncated << block_bits;
+
+ return ((blocks_truncated - 1) << block_bits) + block_offset;
+}
+
/*
* Calculate the range inside the folio that we actually need to read.
*/
@@ -272,7 +288,8 @@ static void iomap_adjust_read_range(struct inode *inode, struct folio *folio,
/* truncate len if we find any trailing uptodate block(s) */
while (++i <= last) {
if (ifs_block_is_uptodate(ifs, i)) {
- plen -= (last - i + 1) * block_size;
+ plen -= iomap_bytes_to_truncate(*pos + plen,
+ block_bits, last - i + 1);
last = i - 1;
break;
}
@@ -288,7 +305,8 @@ static void iomap_adjust_read_range(struct inode *inode, struct folio *folio,
unsigned end = offset_in_folio(folio, isize - 1) >> block_bits;

if (first <= end && last > end)
- plen -= (last - end) * block_size;
+ plen -= iomap_bytes_to_truncate(*pos + plen, block_bits,
+ last - end);
}

*offp = poff;
--
2.51.0