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

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Date:   Mon, 11 Jan 2021 10:49:20 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     linux-block@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Jonathan Corbet <corbet@....net>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, target-devel@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-doc@...r.kernel.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v3 2/7] bvec/iter: disallow zero-length segment bvecs

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@....de>
> Signed-off-by: Pavel Begunkov <asml.silence@...il.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@...hat.com>

-- 
Ming