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Date: Thu, 26 Feb 2015 15:15:25 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC PATCH 11/14] allow writing to the backing file
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To: Max Reitz <mreitz@redhat.com>, Wen Congyang <wency@cn.fujitsu.com>, qemu devel <qemu-devel@nongnu.org>, Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>, Jiang Yunhong <yunhong.jiang@intel.com>, Dong Eddie <eddie.dong@intel.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>, Gonglei <arei.gonglei@huawei.com>, Yang Hongyang <yanghy@cn.fujitsu.com>, zhanghailiang <zhang.zhanghailiang@huawei.com>



On 23/02/2015 23:03, Max Reitz wrote:
> On 2015-02-11 at 22:07, Wen Congyang wrote:
>> Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
>> Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
>> Signed-off-by: Gonglei <arei.gonglei@huawei.com>
>> ---
>>   block.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> I don't think this is a good idea. With this patch, every time you open
> a COW file (with a backing file) R/W, the backing file will be writable.
> I'd rather like a way to explicitly overwrite the R/W mode of the
> backing file; but by default, in my opinion, it should stay read-only.

I agree.

Perhaps blkcolo_open or colo_svm_init can take care of setting
BDRV_O_RDWR on the backing file?  They could also use bdrv_reopen.

Paolo

> Max
> 
>> diff --git a/block.c b/block.c
>> index 067c44b..96cf973 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -856,8 +856,8 @@ static int bdrv_inherited_flags(int flags)
>>    */
>>   static int bdrv_backing_flags(int flags)
>>   {
>> -    /* backing files always opened read-only */
>> -    flags &= ~(BDRV_O_RDWR | BDRV_O_COPY_ON_READ);
>> +    /* backing files are opened read-write for block replication */
>> +    flags &= ~BDRV_O_COPY_ON_READ;
>>         /* snapshot=on is handled on the top layer */
>>       flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_TEMPORARY);
>