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

[Brian Foster <bfoster@redhat.com>]

On Thu, Apr 16, 2026 at 11:06:42AM +0800, Fengnan Chang wrote:
> 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));
> +

This seems reasonable to me in principle, but it feels a little odd to
leave a reset helper that doesn't really do a "reset." I wonder if this
should be refactored into an iomap_iter_complete() (i.e. "complete an
iteration") helper that includes the ret assignment logic just above the
reset call and returns it, and then maybe leave a oneline comment above
the memset so somebody doesn't blindly fold it back in the future. So
for example:

	ret = iomap_iter_complete(iter);
	if (ret <= 0)
		return ret;

	/* save cycles and only clear the mappings if we plan to iterate */
	memset(..);
	...

We'd probably have to recheck some of the iter state within the new
helper, but that doesn't seem like a big deal to me. Thoughts?

Brian

>  begin:
>  	ret = ops->iomap_begin(iter->inode, iter->pos, iter->len, iter->flags,
>  			       &iter->iomap, &iter->srcmap);
> -- 
> 2.39.5 (Apple Git-154)
> 
>