iomap: add comments for ifs_clear/set_range_dirty()

The range alignment strategy differs between ifs_clear_range_dirty() and
ifs_set_range_dirty(). The former rounds inwards to clear only
fully-covered blocks, while the latter rounds outwards to mark any
partially-touched block as dirty. Add comments to document this
asymmetry in block range calculation.

Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20260714082325.325163-6-yi.zhang@huaweicloud.com
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 3a3ac3051fb00a4c670b5bb3d3c09347ea8c3404..6d9a2efd4beef635e4aa42474fd2d99792d8aaeb 100644 (file)
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -174,6 +174,13 @@ static unsigned iomap_find_dirty_range(struct folio *folio, u64 *range_start,
        return range_end - *range_start;
 }
 
+/*
+ * Clear the per-block dirty bits for the range [@off, @off + @len) within a
+ * folio.  The range is rounded inwards so that only blocks fully covered by
+ * the range are cleared.  This is required for operations like folio
+ * invalidation, where we must ensure a block is fully clean before discarding
+ * it.
+ */
 static void ifs_clear_range_dirty(struct folio *folio,
                struct iomap_folio_state *ifs, size_t off, size_t len)
 {
@@ -201,6 +208,13 @@ static void iomap_clear_range_dirty(struct folio *folio, size_t off, size_t len)
                ifs_clear_range_dirty(folio, ifs, off, len);
 }
 
+/*
+ * Set the per-block dirty bits for the range [@off, @off + @len) within a
+ * folio.  The range is rounded outwards so that any block partially touched
+ * by the range is marked dirty.  This ensures blocks containing even a
+ * single dirty byte will be included in subsequent writeback, preventing
+ * data loss when partial blocks are written.
+ */
 static void ifs_set_range_dirty(struct folio *folio,
                struct iomap_folio_state *ifs, size_t off, size_t len)
 {