Date: Thu, 9 Sep 2004 15:54:20 +0100
From: Russell King <rmk+lkml@arm.linux.org.uk>
To: Bartlomiej Zolnierkiewicz <bzolnier@elka.pw.edu.pl>
Cc: linux-ide@vger.kernel.org, linux-kernel@vger.kernel.org,
       Jens Axboe <axboe@suse.de>, Jeff Garzik <jgarzik@pobox.com>
Subject: Re: [patch][9/9] block: remove bio walking
Message-ID: <20040909155420.D6434@flint.arm.linux.org.uk>
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	linux-ide@vger.kernel.org, linux-kernel@vger.kernel.org,
	Jens Axboe <axboe@suse.de>, Jeff Garzik <jgarzik@pobox.com>
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On Thu, Sep 09, 2004 at 04:28:25PM +0200, Bartlomiej Zolnierkiewicz wrote:
> On Thursday 09 September 2004 16:04, Russell King wrote:
> > On Thu, Sep 09, 2004 at 03:53:13PM +0200, Bartlomiej Zolnierkiewicz wrote:
> > > On Thursday 09 September 2004 10:03, Russell King wrote:
> > > > On Wed, Sep 08, 2004 at 09:27:04PM +0200, Bartlomiej Zolnierkiewicz wrote:
> > > > > [patch] block: remove bio walking
> > > > > 
> > > > > IDE driver was the only user of bio walking code.
> > > 
> > > was in -bk10 :-(
> > > 
> > > > The MMC driver also uses this.  Please don't remove.
> > > 
> > > OK I'll just drop this patch but can't we also use scatterlists in MMC?
> > > 
> > > The point is that I now think bio walking was a mistake and accessing
> > > bios directly from low-level drivers is a layering violation (thus
> > > all the added complexity). Moreover with fixed IDE PIO and without
> > > bio walking code it should be possible to shrink struct request by
> > > removing all "current" entries.
> > 
> > I'm wondering whether it is legal to map onto SG lists and then do PIO.
> > Provided we don't end up using the DMA API and then using PIO to the
> > original pages, it should work.
> 
> Yes, it actually works fine. See the other patches from the patchkit. :-)

Actually, if you've only tested x86, you don't know if it works fine.
x86 is a rather benign architecture when it comes to testing whether
various interfaces are being used correctly.

> > However, using the SG lists does finally provide us with a nice way to
> > ensure that we have the right information to finally fix IDE wrt the
> > PIO cache issues (dirty cache lines being left in the page cache.)
> 
> Could you explain the issue a bit more?

Essentially, kernel PIO writes data into the page cache, and that action
may leave data in the CPU's caches.  Since the kernels mappings may not
be coherent with mappings in userspace, data written to the kernel
mappings may remain in the data cache, and stale data would be visible
to user space.

There has been talk about using flush_dcache_page() to resolve
this issue, but I'm not sure what the outcome was.  Certainly
flush_dcache_page() is supposed to be used before the data in the
kernels page cache is read or written.

See Documentation/cachetlb.txt for further information on this
interface.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 PCMCIA      - http://pcmcia.arm.linux.org.uk/
                 2.6 Serial core
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