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Date: Wed, 6 Aug 2025 16:53:34 +0200
From: Christoph Hellwig <hch@lst.de>
To: Keith Busch <kbusch@kernel.org>
Cc: Keith Busch <kbusch@meta.com>, linux-block@vger.kernel.org,
	linux-nvme@lists.infradead.org, hch@lst.de, axboe@kernel.dk
Subject: Re: [PATCH 1/2] block: accumulate segment page gaps per bio
Message-ID: <20250806145333.GA20102@lst.de>
References: <20250805195608.2379107-1-kbusch@meta.com> <aJKLGy9UkVjJTIIQ@kbusch-mbp>
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On Tue, Aug 05, 2025 at 04:52:11PM -0600, Keith Busch wrote:
> On Tue, Aug 05, 2025 at 12:56:07PM -0700, Keith Busch wrote:
> > +	bv = next->bio->bi_io_vec[0];
> > +	bvprv = req->biotail->bi_io_vec[req->biotail->bi_vcnt - 1];
> 
> Hm, commit 7bcd79ac50d9d8 suggests I missed something obvious about not
> being allowed to use 'bi_vcnt - 1' for the tail bvec of the previous
> bio. I guess it's some stacking thing.

It is bio cloning and splitting.  bi_io_vec is the original payload added
to a bio, and directly accessing it is only allowed for the submitter,
i.e. usually the file system.  The I/O stack always need to go through
the bio_iter to deal with split/cloned/truncated/partially completed and
resubmitted bios.