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Date: Fri, 22 May 2026 16:08:56 -0600
From: Keith Busch <kbusch@kernel.org>
To: Ming Lei <tom.leiming@gmail.com>
Cc: Keith Busch <kbusch@meta.com>, axboe@kernel.dk, hch@lst.de,
	linux-block@vger.kernel.org
Subject: Re: [PATCHv3] blk-mq: pop cached request if it is usable
Message-ID: <ahDT-JZZGyyTXyii@kbusch-mbp>
References: <20260521190253.242065-1-kbusch@meta.com>
 <ag-WUymW2vwVyMJB@fedora>
 <ag-1EoopSdp0JZR5@kbusch-mbp>
 <ag_XoloTHEgt3Y8s@fedora>
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In-Reply-To: <ag_XoloTHEgt3Y8s@fedora>

On Fri, May 22, 2026 at 12:12:18PM +0800, Ming Lei wrote:
> On Thu, May 21, 2026 at 07:44:50PM -0600, Keith Busch wrote:
> > On Fri, May 22, 2026 at 07:33:39AM +0800, Ming Lei wrote:
> > > 
> > > BTW, as mentioned in v2, the request may be added back in case of merge,
> > > but seems not a big deal given blk_mq_free_plug_rqs() doesn't free requests
> > > in batch.
> > 
> > We could introduce a special goto label for the merge case to push it
> 
> It can be done simply by replacing the added `blk_mq_free_request` with moving
> it back to plug list.

What I'm worried about is hitting a blocking allocation, then the
cached_rqs list is freed, leaving the current request from it the only
one still holding a queue reference. I think we ought to re-enter the
queue in that case.

I suggested a special goto for the successful merge because a plug merge
couldn't happen if a previous allocation did schedule since that flushes
the plug. I guess we'd have to distinguish a plug merge vs a sched
merge, though.