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Date: Tue, 28 Jul 2026 06:23:13 +0200
From: Christoph Hellwig <hch@lst.de>
To: yu kuai <yukuai@fygo.io>
Cc: axboe@kernel.dk, tj@kernel.org, hch@lst.de, dongsheng.yang@linux.dev,
	cengku@gmail.com, josef@toxicpanda.com, nilay@linux.ibm.com,
	ming.lei@redhat.com, linux-block@vger.kernel.org,
	cgroups@vger.kernel.org
Subject: Re: [RFC PATCH v2 1/8] block: associate blkg in submit_bio instead
 of bio_set_dev
Message-ID: <20260728042313.GA19817@lst.de>
References: <20260724123037.3004560-1-yukuai@kernel.org> <20260724123037.3004560-2-yukuai@kernel.org> <933b4285-1c85-4076-831a-0501d5c38e16@fygo.io>
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On Mon, Jul 27, 2026 at 04:16:29PM +0800, yu kuai wrote:
> > open-coded blkg_put().
> 
> Turns out bio will be submitted by kworker for some drivers and for
> blkcg_punt_bio_submit(). Is it possible to record blkcg during bio initialization,
> as blkcg must exist, and then covert it to blkg during submission? I can use union
> for blkcg and blkg to avoid new field in struct bio.

Mixing the two up sounds like a receipt for a lot of problems
unfortunately.  I wonder if we could switch to only store the blkcg in
the bio and look up the blkg as needed.  This assumes we can get away
with a relatively small numbers of lookups for typical setups.  I'm not
really sure how much the different users are combined - if not much this
might actually be a big net benefit, if on the other hand we'd have to
do a lot of separate lookups it might not work very well.

This could be combined with a way to do more efficient blkg lookups
using e.g. a rhashtable.