Date: Wed, 30 Nov 2005 07:12:21 +0200 (EET)
From: Kai Makisara <Kai.Makisara@kolumbus.fi>
To: Ryan Richter <ryan@tau.solarneutrino.net>
cc: Andrew Morton <akpm@osdl.org>, linux-scsi@vger.kernel.org,
       linux-kernel@vger.kernel.org
Subject: Re: Fw: crash on x86_64 - mm related?
In-Reply-To: <Pine.LNX.4.63.0511292239070.5739@kai.makisara.local>
Message-ID: <Pine.LNX.4.63.0511300646150.24439@kai.makisara.local>
References: <20051129092432.0f5742f0.akpm@osdl.org>
 <Pine.LNX.4.63.0511292147120.5739@kai.makisara.local>
 <20051129203112.GD6326@tau.solarneutrino.net> <Pine.LNX.4.63.0511292239070.5739@kai.makisara.local>
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On Tue, 29 Nov 2005, Kai Makisara wrote:

> On Tue, 29 Nov 2005, Ryan Richter wrote:
> 
> > On Tue, Nov 29, 2005 at 10:04:39PM +0200, Kai Makisara wrote:
> > > I looked at the driver and it seems that there is a bug: st_write calls 
> > > release_buffering at the end even when it has started an asynchronous 
> > > write. This means that it releases the mapping while it is being used!
> > > (I wonder why this has not been noticed earlier.)
> > > 
> > > The patch below (against 2.6.15-rc2) should fix this bug and some others 
> > > related to buffering. It is based on the patch "[PATCH] SCSI tape direct 
> > > i/o fixes" I sent to linux-scsi on Nov 21. The patch restores setting 
> > > pages dirty after reading and clears number of s/g segments when the 
> > > pointers are not valid any more.
> > > 
> > > The patch has been lightly tested with AMD64.
> > 
> > This applies cleanly to 2.6.14.2, do you forsee any problems using it
> > with that kernel?  I'd like to not change too many things at once.
> > 
> No, I don't see any potential problems applying this patch to 2.6.14.2. 
> There is nothing specific to 2.6.15-rc2.
> 
> If someone sees that there is something wrong, please yell. The 
> main purpose of the patch is not to call release_buffering() at the end of 
> st_write() when starting asynchronous write and call it in 
> write_behind_check() instead.
> 
Yelling!

The patch does not cause any problems but my theory about calling 
release_buffering() too early is wrong: asynchronous writes are not done 
with direct i/o. (The reason for this choice is that the API allows the 
user to do anything with the buffer between writes and so the SCSI write 
has to be finished before returning from write().)

The other fixes in the patch are valid but I don't see how they should fix 
this problem.

-- 
Kai
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