linux-kernel - Re: [PATCH] scsi: virtio-scsi: Fix address translation failure of HighMem pages used by sg list

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Date:	Fri, 27 Jul 2012 11:12:58 +0800
From:	Wang Sen <kelvin.xupt@...il.com>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Wang Sen <senwang@...ux.vnet.ibm.com>,
	linux-scsi@...r.kernel.org, JBottomley@...allels.com,
	stefanha@...ux.vnet.ibm.com, mc@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scsi: virtio-scsi: Fix address translation failure of
 HighMem pages used by sg list

2012/7/25 Boaz Harrosh <bharrosh@...asas.com>:
> On 07/25/2012 02:44 PM, Sen Wang wrote:
>
>> 2012/7/25 Paolo Bonzini <pbonzini@...hat.com>:
>>> Il 25/07/2012 10:29, Wang Sen ha scritto:
>>>> When using the commands below to write some data to a virtio-scsi LUN of the
>>>> QEMU guest(32-bit) with 1G physical memory(qemu -m 1024), the qemu will crash.
>>>>
>>>>       # sudo mkfs.ext4 /dev/sdb  (/dev/sdb is the virtio-scsi LUN.)
>>>>       # sudo mount /dev/sdb /mnt
>>>>       # dd if=/dev/zero of=/mnt/file bs=1M count=1024
>>>>
>>>> In current implementation, sg_set_buf is called to add buffers to sg list which
>>>> is put into the virtqueue eventually. But there are some HighMem pages in
>>>> table->sgl can not get virtual address by sg_virt. So, sg_virt(sg_elem) may
>>>> return NULL value. This will cause QEMU exit when virtqueue_map_sg is called
>>>> in QEMU because an invalid GPA is passed by virtqueue.
>>>
>>> Heh, I was compiling (almost) the same patch as we speak. :)
>>
>> Uh, what a coincidence! :)
>>
>>>
>>> I've never seen QEMU crash; the VM would more likely just fail to boot
>>> with a panic.  But it's the same bug anyway.
>>
>> I never met this before. How this situation happens?
>>
>>>
>>>> My solution is using sg_set_page instead of sg_set_buf.
>>>>
>>>> I have tested the patch on my workstation. QEMU would not crash any more.
>>>>
>>>> Signed-off-by: Wang Sen <senwang@...ux.vnet.ibm.com>
>>>> ---
>>>>  drivers/scsi/virtio_scsi.c |    3 ++-
>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
>>>> index 1b38431..fc5c88a 100644
>>>> --- a/drivers/scsi/virtio_scsi.c
>>>> +++ b/drivers/scsi/virtio_scsi.c
>>>> @@ -198,7 +198,8 @@ static void virtscsi_map_sgl(struct scatterlist *sg, unsigned int *p_idx,
>>>>       int i;
>>>>
>>>>       for_each_sg(table->sgl, sg_elem, table->nents, i)
>>>> -             sg_set_buf(&sg[idx++], sg_virt(sg_elem), sg_elem->length);
>>>> +             sg_set_page(&sg[idx++], sg_page(sg_elem), sg_elem->length,
>>>> +                     sg_elem->offset);
>>>
>>> This can simply be
>>>
>>>    sg[idx++] = *sg_elem;
>>>
>>
>> Yes, I saw your another E-mail. I think you're right. Simply calling
>> sg_set_page can not handle
>> the flag bits correctly. So, I'll repost the patch soon. Thank you!
>>
>
>
> No this code is correct, though you will need to make sure to properly
> terminate the destination sg_list.

Yes, the terminate marker in the destination list is set when initialization.
sg_set_page would not break this marker because it saved both the two
maker bits at sg_asign_page.

Also, the allocation of destination sg list considered the total number of
the source sg_list. So, sg_set_page can work correctly.

The value assignment method also can also work correctly, because the
termination marker in source sg_list has been set in blk_rq_map_sg(), as
the last entry of source sg_list is just copied to the the last entry
in destination
list.

Uh, Paolo, Boaz, have  you reached agreement on which method to use?
>
> But since old code was using sg_set_buf(), than it means it was properly
> terminated before, and there for this code is good as is please don't
> touch it.
>
> Thanks
> Boaz
>
>>> Can you repost it with this change, and also add stable@...r.kernel.org
>>> to the Cc?  Thanks very much!
>>>
>>> Paolo
>>> --
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>>
>>
>>
>



-- 
------------------------------------------
Wang Sen
Addr: XUPT,Xi'an,Shaanxi,China
Email: kelvin.xupt@...il.com
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