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Date: Tue, 30 Jan 2024 15:43:13 +0100
From: Christoph Hellwig <hch@lst.de>
To: John Garry <john.g.garry@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	Damien Le Moal <dlemoal@kernel.org>,
	Keith Busch <kbusch@kernel.org>, Sagi Grimberg <sagi@grimberg.me>,
	linux-block@vger.kernel.org, linux-nvme@lists.infradead.org,
	virtualization@lists.linux.dev, Hannes Reinecke <hare@suse.de>
Subject: Re: [PATCH 04/14] block: use queue_limits_commit_update in
 queue_max_sectors_store
Message-ID: <20240130144313.GB32125@lst.de>
References: <20240128165813.3213508-1-hch@lst.de> <20240128165813.3213508-5-hch@lst.de> <e65301d3-7274-45fa-a64c-5039257aa333@oracle.com>
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On Tue, Jan 30, 2024 at 12:14:32PM +0000, John Garry wrote:
> On 28/01/2024 16:58, Christoph Hellwig wrote:
>> Convert queue_max_sectors_store to use queue_limits_commit_update to
>> check and update the max_sectors limit and freeze the queue before
>> doing so to ensure we don't have requests in flight while changing
>> the limits.
>>
>> Note that this removes the previously held queue_lock that doesn't
>> protect against any other reader or writer.
>
> I don't really get why we specifically locked that code segment in 
> queue_max_sectors_store() previously. Was it to ensure max_sectors and 
> q->disk->bdi->io_pages are always atomically updated?

It's been there basically forever.  Back in the day before blk-mq
and lock splitting it might actually have protected something.