iomap: avoid unnecessary ifs_set_range_uptodate() with locks

In the buffer write path, iomap_set_range_uptodate() is called every
time iomap_end_write() is called. But if folio_test_uptodate() holds, we
know that all blocks in this folio are already in the uptodate state, so
there is no need to go deep into the critical section of state_lock to
execute bitmap_set().

This is because the folios always creep towards ifs_is_fully_uptodate()
state and once they've gotten there folio_mark_uptodate() is called, which
means the folio is uptodate.

Then once a folio is uptodate, there is no route back to !uptodate without
going through the removal of the folio from the page cache. Therefore, it's
fine to use folio_test_uptodate() to short-circuit unnecessary code paths.

Although state_lock may not have significant lock contention due to
folio lock, this patch at least reduces the number of instructions,
especially the expensive lock-prefixed instructions.

Signed-off-by: Jinliang Zheng <[email protected]>
Link: https://lore.kernel.org/[email protected]
Reviewed-by: Darrick J. Wong <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: fatherMatrix

fs/iomap/buffered-io.c

@@ -71,6 +71,9 @@ static void iomap_set_range_uptodate(struct folio *folio, size_t off,
 	unsigned long flags;
 	bool uptodate = true;
 
+	if (folio_test_uptodate(folio))
+		return;
+
 	if (ifs) {
 		spin_lock_irqsave(&ifs->state_lock, flags);
 		uptodate = ifs_set_range_uptodate(folio, ifs, off, len);