[PATCH] iomap: avoid memset iomap when iter is done

[Fengnan Chang <fengnanchang@gmail.com>]

When iomap_iter() finishes its iteration (returns <= 0), it is no longer
necessary to memset the entire iomap and srcmap structures.

In high-IOPS scenarios (like 4k randread NVMe polling with io_uring),
where the majority of I/Os complete in a single extent map, this wasted
memory write bandwidth, as the caller will just discard the iterator.

Use this command to test:
taskset -c 30 ./t/io_uring -p1 -d512 -b4096 -s32 -c32 -F1 -B1 -R1 -X1
-n1 -P1 /mnt/testfile
IOPS improve about 5% on ext4 and XFS.

However, we MUST still call iomap_iter_reset_iomap() to release the
folio_batch if IOMAP_F_FOLIO_BATCH is set, otherwise we leak page
references. Therefore, split the cleanup logic: always release the
folio_batch, but skip the memset() when ret <= 0.

Signed-off-by: Fengnan Chang <changfengnan@bytedance.com>
---
 fs/iomap/iter.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
index c04796f6e57f..91eb5e6165ff 100644
--- a/fs/iomap/iter.c
+++ b/fs/iomap/iter.c
@@ -15,8 +15,6 @@ static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
 	}
 
 	iter->status = 0;
-	memset(&iter->iomap, 0, sizeof(iter->iomap));
-	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
 }
 
 /* Advance the current iterator position and decrement the remaining length */
@@ -106,6 +104,9 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
 	if (ret <= 0)
 		return ret;
 
+	memset(&iter->iomap, 0, sizeof(iter->iomap));
+	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
+
 begin:
 	ret = ops->iomap_begin(iter->inode, iter->pos, iter->len, iter->flags,
 			       &iter->iomap, &iter->srcmap);
-- 
2.39.5 (Apple Git-154)