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Date:      Thu, 16 Apr 2009 17:48:07 -0400
From:      John Baldwin <jhb@freebsd.org>
To:        Scott Long <scottl@samsco.org>
Cc:        Damian Gerow <dgerow@afflictions.org>, freebsd-current@freebsd.org, Richard Todd <rmtodd@ichotolot.servalan.com>
Subject:   Re: ZFS checksum errors on umass(4) insertion
Message-ID:  <200904161748.08402.jhb@freebsd.org>
In-Reply-To: <49E7A404.5090208@samsco.org>
References:  <49BD117B.2080706@163.com> <200904161624.51920.jhb@freebsd.org> <49E7A404.5090208@samsco.org>

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On Thursday 16 April 2009 5:32:52 pm Scott Long wrote:
> John Baldwin wrote:
> > Can you please try http://www.FreeBSD.org/~jhb/patches/dma_pg.patch?  This
> > lines up with your analysis in that it fixes a problem in the bounce 
buffer
> > code that was introduced with the new USB stack (and only triggers when 
the
> > USB code has to use a bounce buffer).
> > 
> 
> As a data point, most normal I/O is not going to trigger this bug, even
> if it gets bounced.  I/O using O_DIRECT can, and GEOM discovery I/O can
> as well.  Since memory is allocated from the top of the system, I think
> that the damage gets done early during boot, and then propagates out
> over time as the system becomes busier.

Hmm, are you sure regular I/O won't trigger it as well?  All it takes is for 
any USB transfer that starts off within a page to get a page into a non-zero 
offset and later have a request >= PAGE_SIZE bounce.  Since the VM is going 
to always ask for I/O to pages (e.g. GET/PUTPAGES) normal disk I/O would 
break if it uses the bad bounce page I think.

-- 
John Baldwin



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