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Date: Tue, 12 Mar 2024 22:06:05 +0100
From: Christoph Hellwig <hch@lst.de>
To: Keith Busch <kbusch@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	Chandan Babu R <chandanbabu@kernel.org>,
	linux-block@vger.kernel.org, linux-nvme@lists.infradead.org,
	linux-xfs@vger.kernel.org
Subject: Re: [PATCH 2/5] block: add a bio_chain_and_submit helper
Message-ID: <20240312210605.GA1500@lst.de>
References: <20240312144532.1044427-1-hch@lst.de> <20240312144532.1044427-3-hch@lst.de> <ZfBr91m4oQS_VYFg@kbusch-mbp.mynextlight.net>
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On Tue, Mar 12, 2024 at 08:51:35AM -0600, Keith Busch wrote:
> > +
> > +struct bio *blk_next_bio(struct bio *bio, struct block_device *bdev,
> > +		unsigned int nr_pages, blk_opf_t opf, gfp_t gfp)
> > +{
> > +	return bio_chain_and_submit(bio, bio_alloc(bdev, nr_pages, opf, gfp));
> > +}
> 
> I realize you're not changing any behavior here, but I want to ask, is
> bio_alloc() always guaranteed to return a valid bio? It sure looks like
> it can return NULL under some uncommon conditions, but I can't find
> anyone checking the result. So I guess it's safe?

bio_alloc can only fail if we don't wait for allocations, that is if
__GFP_DIRECT_RECLAIM isn't set.

We could an assert here.  Or work on killing the gfp_flags argument
and just add a bio_alloc_nowait for the few cases that need it.