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Date: Tue, 18 Nov 2025 16:10:48 -0700
From: Keith Busch <kbusch@kernel.org>
To: Christoph Hellwig <hch@lst.de>
Cc: Leon Romanovsky <leon@kernel.org>, Jens Axboe <axboe@kernel.dk>,
	Sagi Grimberg <sagi@grimberg.me>, linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
	Chaitanya Kulkarni <kch@nvidia.com>
Subject: Re: [PATCH v2 1/2] nvme-pci: Use size_t for length fields to handle
 larger sizes
Message-ID: <aRz8-IB6dq5tlpae@kbusch-mbp>
References: <20251117-nvme-phys-types-v2-0-c75a60a2c468@nvidia.com>
 <20251117-nvme-phys-types-v2-1-c75a60a2c468@nvidia.com>
 <aRt5DPnMwzDw8_dF@kbusch-mbp>
 <20251118051823.GA21858@lst.de>
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In-Reply-To: <20251118051823.GA21858@lst.de>

On Tue, Nov 18, 2025 at 06:18:23AM +0100, Christoph Hellwig wrote:
> On Mon, Nov 17, 2025 at 12:35:40PM -0700, Keith Busch wrote:
> > > +	size_t total_len;
> > 
> > Changing the generic phys_vec sounds fine, but the nvme driver has a 8MB
> > limitation on how large an IO can be, so I don't think the driver's
> > length needs to match the phys_vec type.
> 
> With the new dma mapping interface we could lift that limits for
> SGL-based controllers as we basically only have a nr_segments limit now.
> Not that I'm trying to argue for multi-GB I/O..

It's not a bad idea. The tricky part is in the timeout handling. If
we allow very large IO, I think we need a dynamic timeout value to
account for the link's throughput. We can already trigger blk-mq
timeouts if you saturate enough queues with max sized IO, despite
everything else working-as-designed.