Re: [Qemu-devel] [PATCH 1/7] block: only force IO completion in .bdrv_truncate if we are shrinking

[Peter Lieven]

Am 07.03.2013 um 10:03 schrieb Kevin Wolf <address@hidden>:

> Am 07.03.2013 um 09:56 hat Peter Lieven geschrieben:
>> On 07.03.2013 09:50, Kevin Wolf wrote:
>>> Am 06.03.2013 um 21:39 hat Paolo Bonzini geschrieben:
>>>> Il 06/03/2013 20:03, Peter Lieven ha scritto:
>>>>> Am 06.03.2013 19:48, schrieb Jeff Cody:
>>>>>> On Wed, Mar 06, 2013 at 07:31:51PM +0100, Paolo Bonzini wrote:
>>>>>>> Il 06/03/2013 19:14, Jeff Cody ha scritto:
>>>>>>>> QCOW breaks with it using a normal raw posix file as a device.  As a
>>>>>>>> test: qemu-img create -f qcow test.qcow 5G.  Now run qemu with that
>>>>>>>> drive mounted, and try to partition and format it.  QEMU now asserts.
>>>>>>>> 
>>>>>>>> The nicety of being able to using truncate during a write call,
>>>>>>>> especially for VHDX (which can have relatively large block/cluster
>>>>>>>> sizes), so to grow the file sparsely in a dynamically allocated file.
>>>>>>> 
>>>>>>> Perhaps we need two APIs, "truncate" and "revalidate".
>>>>>>> 
>>>>>>> Truncate should be a no-op if (!bs->growable).
>>>>>>> 
>>>>>>> Revalidate could be called by the block_resize monitor command with no
>>>>>>> size specified.
>>>>>>> 
>>>>>>> Paolo
>>>>>> 
>>>>>> I think that is a good solution.  Is it better to have "truncate" and
>>>>>> "revalidate", or "truncate" and "grow", with grow being a subset of
>>>>>> truncate, with fewer restrictions?  There may still be operations
>>>>>> where it is OK to grow a file, but not OK to shrink it.
>>> 
>>> What semantics would the both operations have? Is truncate the same as
>>> it used to be? I don't really understand what "revalidate" would do, it
>>> sounds like a read-only operation from its name?
>>> 
>>>>> Or as a first step:
>>>>> 
>>>>> a) Call brdv_drain_all() only if the device is shrinked (independently of 
>>>>> !bs->growable)
>>>>> b) Call brdv_drain_all() inside iscsi_truncate() because it is a special 
>>>>> requirement there
>>>>> c) Fix the value of bs->growable for all drivers
>>>> 
>>>> Let's start from (c).  bdrv_file_open sets bs->growable = 1.  I think it
>>>> should be removed and only the file protocol should set it.
>>> 
>>> This is probably right.
>> 
>> If bs->growable is 1 for all drivers, whats the fix status of CVE-2008-0928? 
>> This
>> flag was introduced as a fix for this problem.
>> 
>> bdrv_check_byte_request() does nothing useful if bs->growable is 1.
> 
> Don't ignore the difference between bdrv_open() and bdrv_file_open().
> Typically you have two BDSes: On top there is e.g. a qcow2 BDS that is
> opened through bdrv_open() and has bs->growable = false. Its bs->file is
> using the file protocol (raw-posix driver) and opened by
> bdrv_file_open(). This one has bs->file->growable = true so that qcow2
> can write to newly allocated areas without calling bdrv_truncate()
> first.

Sorry, I have to admin I am little confused by what is happening in bdrv_open().

However, what I can say is that bs->growable is 1 for an iSCSI backed
harddrive and I wonder how this can happen if bdrv_file_open is not used for
opening it because that is the only place where bs->growable is set to 1.

cmdline:
x86_64-softmmu/qemu-system-x86_64 -k de -enable-kvm -m 1024 -drive 
if=virtio,file=iscsi://172.21.200.31/iqn.2001-05.com.equallogic:0-8a0906-16470e107-713001aa6de511e0-001-test/0
 -vnc :1 -boot dc -monitor stdio

Peter

> 
> Kevin