linux-kernel - Re: [ 38/48] SCSI & usb-storage: add try_rc_10

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Message-ID: <Pine.LNX.4.44L0.1207031618450.29825-100000@netrider.rowland.org>
Date:	Tue, 3 Jul 2012 16:25:05 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
cc:	Matthew Wilcox <matthew@....cx>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hans de Goede <hdegoede@...hat.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"alan@...rguk.ukuu.org.uk" <alan@...rguk.ukuu.org.uk>,
	Matthew Dharm <mdharm-usb@...-eyed-alien.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [ 38/48] SCSI & usb-storage: add try_rc_10_first flag

On Wed, 4 Jul 2012, James Bottomley wrote:

> > The reason for the try_rc_10_first flag is that some devices return
> > bogus data in response to RC16.  Like, an 800 GB device claiming to
> > have 3 exabytes.
> 
> So we could easily send both and only believe RC10 if the device is
> under 2TB.

More precisely, if RC10 indicates the device is under 2 TB then ignore 
the RC16 capacity.  Or maybe issue a warning if it disagrees with the 
RC10 capacity.

>  However, what about all the extra flags we read out of RC16,
> like trim, large sector size and DIF capability?  If the device lies
> about its capacity, won't we get bogus values for those as well, which
> is going to cause other screw ups?

That seems quite likely.  I don't know the answer for any individual 
cases, however.

For the greatest safety, I suppose we could disbelieve all the fields 
in the RC16 response if the RC16 capacity disagrees with the RC10 
capacity and the RC10 capacity is under 2 TB.

Alan Stern

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