iomap: correct the range of a partial dirty clear

[ Upstream commit 88c2651 ]

The block range calculation in ifs_clear_range_dirty() is incorrect when
partially clearing a range in a folio. We cannot clear the dirty bit of
the first block or the last block if the start or end offset is not
blocksize-aligned. This has not yet caused any issues since we always
clear a whole folio in iomap_writeback_folio().

Fix this by rounding up the first block to blocksize alignment, and
calculate the last block by rounding down (using truncation). Correct
the nr_blks calculation accordingly.

Fixes: 4ce02c6 ("iomap: Add per-block dirty state tracking to improve performance")
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20260714082325.325163-2-yi.zhang@huaweicloud.com
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: zhangyi089

fs/iomap/buffered-io.c

@@ -137,13 +137,17 @@ static void ifs_clear_range_dirty(struct folio *folio,
 {
 	struct inode *inode = folio->mapping->host;
 	unsigned int blks_per_folio = i_blocks_per_folio(inode, folio);
-	unsigned int first_blk = (off >> inode->i_blkbits);
-	unsigned int last_blk = (off + len - 1) >> inode->i_blkbits;
-	unsigned int nr_blks = last_blk - first_blk + 1;
+	unsigned int first_blk = round_up(off, i_blocksize(inode)) >>
+				 inode->i_blkbits;
+	unsigned int last_blk = (off + len) >> inode->i_blkbits;
 	unsigned long flags;
 
+	if (first_blk >= last_blk)
+		return;
+
 	spin_lock_irqsave(&ifs->state_lock, flags);
-	bitmap_clear(ifs->state, first_blk + blks_per_folio, nr_blks);
+	bitmap_clear(ifs->state, first_blk + blks_per_folio,
+		     last_blk - first_blk);
 	spin_unlock_irqrestore(&ifs->state_lock, flags);
 }