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Date: Mon, 23 Feb 2015 17:03:07 -0500
From: Max Reitz <mreitz@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: Wen Congyang <wency@cn.fujitsu.com>, qemu devel <qemu-devel@nongnu.org>, Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@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 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.

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);