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Date: Thu, 8 May 2025 07:03:17 +0200
From: Christoph Hellwig <hch@lst.de>
To: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, Nilay Shroff <nilay@linux.ibm.com>
Subject: Re: [PATCH 1/2] block: move queue quiesce into elevator_change()
Message-ID: <20250508050317.GA27049@lst.de>
References: <20250507120406.3028670-1-ming.lei@redhat.com> <20250507120406.3028670-2-ming.lei@redhat.com> <20250507135349.GA1019@lst.de> <aBtuCDKTQK1PReDg@fedora>
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On Wed, May 07, 2025 at 10:28:24PM +0800, Ming Lei wrote:
> On Wed, May 07, 2025 at 03:53:49PM +0200, Christoph Hellwig wrote:
> > On Wed, May 07, 2025 at 08:04:02PM +0800, Ming Lei wrote:
> > > blk_mq_freeze_queue() can't be called on quiesced queue, otherwise it may
> > > never return if there is any queued requests.
> > > 
> > > Fix it by moving queue quiesce int elevator_change() by adding one flag to
> > > 'struct elv_change_ctx' for controlling this behavior.
> > 
> > Why do we even need to quiesce the queue here, and not anywhere else?
> 
> Quiesce is for draining the in-progress critical area, which can't be
> covered by queue freeze.

I know.  But why do we care about that for removing a scheduler, but
not for changing it?