Re: Fwd: SWIOTLB on 32-bit PAE

[Christian Melki]

Yes. Operator error indeed.

Thanks for asking. Included is the same patch with the SoB.

On 10/05/2015 04:21 PM, Konrad Rzeszutek Wilk wrote:
> On Mon, Oct 05, 2015 at 12:44:55PM +0200, Christian Melki wrote:
> > Joerg,
> >
> > I already sent the patch for Kconfig change to the list.
> > But I discovered something even more disturbing which I mailed Konrad about.
> > With the change my machine does not hang any more, but instead corrupts data
> > which is written. The data however is readable on the machine which
> > corrupted it, unreadable on the other machine without the patch. I don't
> > understand what is going on. I have never encountered "selective" file
> > corruption of fs on different machines. The fs itself checks out cleanly
> > with badblock control (ext4). I fail to understand what is happening.
>
> And I believe this ended up being an operator error?
>
> With the patch you sent, you didn't include your SoB
> (Signed-off-By). I would like to include your patch but I cannot
> with your SoB. It is matter of reading to Documentation/SubmittingPatches
> and just consenting to the Certificate in there.
>
> If you are uncomfortable with that - that is OK, I can make this
> patch myself (a one liner :-)) but I figured it owuld be nice
> for you to have all the credit.
> > Regards,
> > Christian
> >
> > -------- Forwarded Message --------
> > Subject: SWIOTLB on 32-bit PAE
> > Date: Mon, 5 Oct 2015 10:31:39 +0200
> > From: Christian Melki <christian.melki@xxxxxxxxxx>
> > To: linux-kernel@xxxxxxxxxxxxxxx
> > CC: konrad.wilk@xxxxxxxxxx
> >
> > Hi,
> >
> > I discovered that my 32-bit PAE 4.2.0 kernel (no IOMMU code) would hang
> > when writing to my USB disk. The kernel spews million(-ish messages per
> > sec) to syslog, effectively "hanging" userspace with my kernel.
> >
> > Oct  2 14:33:06 voodoochild kernel: [  223.287447] nommu_map_sg:
> > overflow 25dcac000+1024 of device mask ffffffff
> > Oct  2 14:33:06 voodoochild kernel: [  223.287448] nommu_map_sg:
> > overflow 25dcac000+1024 of device mask ffffffff
> > Oct  2 14:33:06 voodoochild kernel: [  223.287449] nommu_map_sg:
> > overflow 25dcac000+1024 of device mask ffffffff
> > ... etc ...
> >
> > In my kernel config I noticed that SWIOTLB was not on. It seems SWIOTLB
> > is provided for 64-bit and 32-bit with IOMMU/AGPGART code. But if I
> > compiled the kernel with PAE and no IOMMU and no other GART code, I
> > would not get SWIOTLB. I'd like to think that SWIOTLB should be selected
> > for 32-bit PAE as default.
> >
> > I have attached a oneliner patch which does that.
> > The patch works for me. The issue where the kernel more or less runs
> > endless bashing of (nommu_?)map_sg when failing is another problem I
> > guess. I expected the kernel/drivers to have some more graceful
> > handling, but I know to little about this area to have a proper opinion.
> >
> > Regards,
> > Christian
>
> > diff -urN linux-4.2.orig/arch/x86/Kconfig linux-4.2/arch/x86/Kconfig
> > --- linux-4.2.orig/arch/x86/Kconfig	2015-10-05 08:56:58.933313678 +0200
> > +++ linux-4.2/arch/x86/Kconfig	2015-10-05 09:00:00.916306025 +0200
> > @@ -1282,6 +1282,7 @@
> >   config X86_PAE
> >   	bool "PAE (Physical Address Extension) Support"
> >   	depends on X86_32 && !HIGHMEM4G
> > +	select SWIOTLB
> >   	---help---
> >   	  PAE is required for NX support, and furthermore enables
> >   	  larger swapspace support for non-overcommit purposes. It

diff -urN linux-4.2.orig/arch/x86/Kconfig linux-4.2/arch/x86/Kconfig
--- linux-4.2.orig/arch/x86/Kconfig	2015-10-05 08:56:58.933313678 +0200
+++ linux-4.2/arch/x86/Kconfig	2015-10-05 09:00:00.916306025 +0200
@@ -1282,6 +1282,7 @@
 config X86_PAE
 	bool "PAE (Physical Address Extension) Support"
 	depends on X86_32 && !HIGHMEM4G
+	select SWIOTLB
 	---help---
 	  PAE is required for NX support, and furthermore enables
 	  larger swapspace support for non-overcommit purposes. It

Signed-off-by: Christian Melki <christian.melki@xxxxxxxxxx>