On Tue, Apr 02, 2019 at 09:46:38AM +0800, Ming Lei wrote:
> Commit f6970f83ef79 ("block: don't check if adjacent bvecs in one bio can
> be mergeable") changes bvec merge by only considering two bvecs from
> different bios. However, if the former bio doesn't inlcude any io bvec,
> then the following warning may be triggered:
>
> warning: ‘bvec.bv_offset’ may be used uninitialized in this function
> [-Wmaybe-uninitialized]
>
> In practice, it shouldn't be triggered.
>
> Fixes it by adding check on former bio, the check shouldn't add any cost
> given 'bio->bi_iter' can be hit in cache.
>
> Reported-by: Jens Axboe <[email protected]>
> Fixes: f6970f83ef79 ("block: don't check if adjacent bvecs in one bio can be
> mergeable")
> Signed-off-by: Ming Lei <[email protected]>
> ---
> block/blk-merge.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 8f96d683b577..1fc66d03893f 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -527,7 +527,7 @@ static int __blk_bios_map_sg(struct request_queue *q,
> struct bio *bio,
> struct scatterlist *sglist,
> struct scatterlist **sg)
> {
> - struct bio_vec bvec, bvprv = { NULL };
> + struct bio_vec uninitialized_var(bvec), bvprv = { NULL };
> struct bvec_iter iter;
> int nsegs = 0;
> bool new_bio = false;
> @@ -550,8 +550,10 @@ static int __blk_bios_map_sg(struct request_queue *q,
> struct bio *bio,
> next_bvec:
> new_bio = false;
> }
> - bvprv = bvec;
> - new_bio = true;
> + if (likely(bio->bi_iter.bi_size)) {
> + bvprv = bvec;
> + new_bio = true;
> + }
> }
Looks __blk_recalc_rq_segments() needs the similar fix too, so please
ignore this one, and I will handle them all in one patch.
Thanks,
Ming
