[PATCH v2 31/83] block: rust: add `Segment::truncate`

[Andreas Hindborg]

Add a method that limits the remaining length of a `Segment` without
moving its offset. This complements `Segment::advance`, which can skip
data at the front but cannot trim data at the back, and gives callers a
way to clip a segment to a maximum byte count before handing it to the
existing `copy_to_page` / `copy_from_page` / `zero_page` helpers, which
already bound themselves by `Segment::len()`.

This is needed by rnull's partial bad-block I/O path, which needs to
clamp per-segment work to a sector boundary computed from the bad-block
range.

Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
---
 rust/kernel/block/bio/vec.rs | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/rust/kernel/block/bio/vec.rs b/rust/kernel/block/bio/vec.rs
index 99ab164d4038..61d83a07397f 100644
--- a/rust/kernel/block/bio/vec.rs
+++ b/rust/kernel/block/bio/vec.rs
@@ -81,6 +81,18 @@ pub fn advance(&mut self, count: u32) -> Result {
         Ok(())
     }
 
+    /// Limit the remaining length of the segment.
+    ///
+    /// Shortens the segment to at most `new_len` bytes. If `new_len` is
+    /// greater than or equal to the current remaining length, the segment is
+    /// left unchanged. The offset is not modified, so subsequent copy
+    /// operations still start from the current position.
+    pub fn truncate(&mut self, new_len: u32) {
+        if new_len < self.len() {
+            self.bio_vec.bv_len = new_len;
+        }
+    }
+
     /// Copy data of this segment into `dst_page`.
     ///
     /// Copies data from the current offset to the next page boundary. That is `PAGE_SIZE -

-- 
2.51.2