linux-kernel - Re: [PATCH] BUG: nr_phys_segments cannot be less than nr_hw

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Message-ID: <20081002165857.GL19428@kernel.dk>
Date:	Thu, 2 Oct 2008 18:58:58 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Nikanth Karthikesan <knikanth@...e.de>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Subject: Re: [PATCH] BUG: nr_phys_segments cannot be less than nr_hw_segments

On Thu, Oct 02 2008, James Bottomley wrote:
> The bug would appear to be that we sometimes only look at q->max_sectors
> when deciding on mergability.  Either we have to insist on max_sectors
> <= hw_max_sectors, or we have to start using min(q->max_sectors,
> q->max_hw_sectors) for this.

q->max_sectors MUST always be <= q->max_hw_sectors, otherwise we could
be sending down requests that are too large for the device to handle. So
that condition would be a big bug. The sysfs interface checks for this,
and blk_queue_max_sectors() makes sure that is true as well.

The fixes proposed still look weird. There is no phys vs hw segment
constraints, the request must adhere to the limits set by both. It's
mostly a moot point anyway, as 2.6.28 will get rid of the hw accounting
anyway.

-- 
Jens Axboe

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