Re: [PATCH] 9p/trans_virtio.c: Fix broken zero-copy on vmalloc() buffers

[Richard Yao]

On Jan 30, 2014, at 7:44 PM, David Miller <davem@xxxxxxxxxxxxx> wrote:

> From: David Miller <davem@xxxxxxxxxxxxx>
> Date: Thu, 30 Jan 2014 16:29:26 -0800 (PST)
> 
>> From: Richard Yao <ryao@xxxxxxxxxx>
>> Date: Thu, 30 Jan 2014 13:02:48 -0500
>> 
>>> The 9p-virtio transport does zero copy on things larger than 1024 bytes
>>> in size. It accomplishes this by returning the physical addresses of
>>> pages to the virtio-pci device. At present, the translation is usually a
>>> bit shift.
>>> 
>>> However, that approach produces an invalid page address when we
>>> read/write to vmalloc buffers, such as those used for Linux kernle
>>> modules. This causes QEMU to die printing:
>>> 
>>> qemu-system-x86_64: virtio: trying to map MMIO memory
>>> 
>>> This patch enables 9p-virtio to correctly handle this case. This not
>>> only enables us to load Linux kernel modules off virtfs, but also
>>> enables ZFS file-based vdevs on virtfs to be used without killing QEMU.
>>> 
>>> Also, special thanks to both Avi Kivity and Alexander Graf for their
>>> interpretation of QEMU backtraces. Without their guidence, tracking down
>>> this bug would have taken much longer.
>>> 
>>> Signed-off-by: Richard Yao <ryao@xxxxxxxxxx>
>>> Acked-by: Alexander Graf <agraf@xxxxxxx>
>>> Reviewed-by: Will Deacon <will.deacon@xxxxxxx>
>> 
>> Applied, thanks.
> 
> Actually I had to revert, is_vmalloc_or_malloc_addr() is not exported to
> modules, so this change breaks the build.

Thanks for catching that. I had originally used is_vmalloc_addr() instead of is_vmalloc_or_malloc_addr(), but changed it after realizing this did not correct the problem on all architectures. The is_vmalloc_addr() lives in headers. I will send out a patch to get that symbol exported and resubmit this after it is merged.--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/