Message-ID: <561297D5.8040107@t2data.com>
Date:	Mon, 5 Oct 2015 17:31:33 +0200
From:	Christian Melki <christian.melki@...ata.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	<joro@...tes.org>, <linux-kernel@...r.kernel.org>
Subject: Re: Fwd: SWIOTLB on 32-bit PAE

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@...ata.com>
>> To: linux-kernel@...r.kernel.org
>> CC: konrad.wilk@...cle.com
>>
>> 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
>>
>


View attachment "swiotlb.patch" of type "text/plain" (543 bytes)

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