[PATCH v4 11/23] iomap: correct the range of a partial dirty clear

[Zhang Yi]

From: Zhang Yi <yi.zhang@xxxxxxxxxx>

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.

Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
---
This is modified from:
 https://lore.kernel.org/linux-fsdevel/20240812121159.3775074-2-yi.zhang@xxxxxxxxxxxxxxx/
Changes:
 - Use round_up() instead of DIV_ROUND_UP() to prevent wasted integer
   division.

 fs/iomap/buffered-io.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index d7b648421a70..64351a448a8b 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -176,13 +176,17 @@ 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);
 }
 
-- 
2.52.0