Re: [PATCH v3 2/7] bvec/iter: disallow zero-length segment bvecs

Ming Lei
2021-01-11T02:51:25Z

On Sat, Jan 09, 2021 at 04:02:58PM +0000, Pavel Begunkov wrote:
> zero-length bvec segments are allowed in general, but not handled by bio
> and down the block layer so filtered out. This inconsistency may be
> confusing and prevent from optimisations. As zero-length segments are
> useless and places that were generating them are patched, declare them
> not allowed.
>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
> Documentation/block/biovecs.rst | 2 ++
> Documentation/filesystems/porting.rst | 7 +++++++
> lib/iov_iter.c | 2 --
> 3 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/block/biovecs.rst b/Documentation/block/biovecs.rst
> index 36771a131b56..ddb867e0185b 100644
> --- a/Documentation/block/biovecs.rst
> +++ b/Documentation/block/biovecs.rst
> @@ -40,6 +40,8 @@ normal code doesn't have to deal with bi_bvec_done.
> There is a lower level advance function - bvec_iter_advance() - which takes
> a pointer to a biovec, not a bio; this is used by the bio integrity code.
>
> +As of 5.12 bvec segments with zero bv_len are not supported.
> +
> What's all this get us?
> =======================
>
> diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
> index 867036aa90b8..c722d94f29ea 100644
> --- a/Documentation/filesystems/porting.rst
> +++ b/Documentation/filesystems/porting.rst
> @@ -865,3 +865,10 @@ no matter what. Everything is handled by the caller.
>
> clone_private_mount() returns a longterm mount now, so the proper destructor of
> its result is kern_unmount() or kern_unmount_array().
> +
> +---
> +
> +**mandatory**
> +
> +zero-length bvec segments are disallowed, they must be filtered out before
> +passed on to an iterator.
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 1635111c5bd2..7de304269641 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -72,8 +72,6 @@
> __start.bi_bvec_done = skip; \
> __start.bi_idx = 0; \
> for_each_bvec(__v, i->bvec, __bi, __start) { \
> - if (!__v.bv_len) \
> - continue; \
> (void)(STEP); \
> } \
> }
> --
> 2.24.0
>

Reviewed-by: Ming Lei <ming.lei@redhat.com>

--
Ming