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Date: Wed, 9 Sep 2026 08:28:27 +0200
From: Christoph Hellwig <hch@lst.de>
To: David Howells <dhowells@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>, Keith Busch <kbusch@kernel.org>,
	Hannes Reinecke <hare@kernel.org>, Christoph Hellwig <hch@lst.de>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Paulo Alcantara <pc@manguebit.org>, netfs@lists.linux.dev,
	linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] block: Fix start and length check added to
 iov_iter_extract_bvecs()
Message-ID: <20260909062827.GA29874@lst.de>
References: <1456602.1788783694@warthog.procyon.org.uk>
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On Mon, Sep 07, 2026 at 01:21:34PM +0100, David Howells wrote:
> index 6665372ecf71..5e9f9e2c30b1 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1921,15 +1921,29 @@ ssize_t iov_iter_extract_bvecs(struct iov_iter *iter, struct bio_vec *bv,
>  		unsigned short max_vecs, unsigned mem_align_mask,
>  		iov_iter_extraction_t extraction_flags)
>  {
> -	unsigned long start = (unsigned long)iter_iov_addr(iter);
>  	unsigned short entries_left = max_vecs - *nr_vecs;
>  	unsigned short nr_pages, i = 0;
>  	size_t left, offset, len;
>  	struct page **pages;
>  	ssize_t size;
>  
> -	if ((start | iter_iov_len(iter)) & mem_align_mask)
> +	/*
> +	 * DMA engines typically have both memory address and length alignment
> +	 * requirements, so check these against the alignment mask.  For UBUF,
> +	 * IOVEC and KVEC, only the current segment will be drawn from; for
> +	 * everything else we might draw from multiple segments, so we need to
> +	 * check those too.

I have a bit of a hard time understanding what "draw from" means here?
I guess you mean that iov_iter_extract_pages only operates on a single
iov/kvec/bvec but could touch multiple segments for the others?
Maybe try to explain that without introducing new terminology?