Re: [Qemu-devel] [PATCH] hostmem-file: add a property 'notrunc' to avoid data corruption

[Eduardo Habkost]

On Thu, Oct 20, 2016 at 04:17:35PM +0200, Igor Mammedov wrote:
> On Thu, 20 Oct 2016 11:47:18 -0200
> Eduardo Habkost <address@hidden> wrote:
> 
> > On Thu, Oct 20, 2016 at 09:33:53PM +0800, Haozhong Zhang wrote:
> > > On 10/20/16 11:21 -0200, Eduardo Habkost wrote:  
> > > > On Thu, Oct 20, 2016 at 02:34:12PM +0200, Igor Mammedov wrote:  
> > > > > On Thu, 20 Oct 2016 14:13:01 +0800
> > > > > Haozhong Zhang <address@hidden> wrote:
> > > > >   
> > > > > > If a file is used as the backend of memory-backend-file and its 
> > > > > > size is
> > > > > > not identical to the property 'size', the file will be truncated. 
> > > > > > For a
> > > > > > file used as the backend of vNVDIMM, its data is expected to be
> > > > > > persistent and the truncation may corrupt the existing data.  
> > > > > I wonder if it's possible just skip 'size' property in your case 
> > > > > instead
> > > > > 'notrunc' property. That way if size is not present one'd get actual 
> > > > > size
> > > > > using get_file_size() and set 'size' to it?
> > > > > And if 'size' is provided and 'size' != file_size then error out.  
> > > > 
> > > > I think it is valid to start with a zero-size file and then let
> > > > QEMU extend it.  
> > > 
> > > For vNVDIMM, extending from zero-size file can be valid when a file is
> > > first used. However, it's not valid for the second and following use
> > > of the same file.
> > >   
> > > > But I agree we should: 1) make 'size' optional as
> > > > you suggested; 2) never truncate the file to a smaller size.
> > > >   
> > > 
> > > I will add another patch for this. Is there any way in QEMU to decide
> > > whether a memory-backend-file object is used for vNVDIMM when the
> > > object is being created? Or 'size' can be optional for all kinds of
> > > usages?  
> > 
> > I believe 'size' can be optional for all usage, because at the
> > moment the memory allocation code asks the backend for a memory
> > region, it is supposed to know desired RAM size from the frontend
> > configuration (-numa, -m, or "size" property of pc-dimm).
> 
> nope, currently the size propagates other way around
>  from back-end to front-end and not backwards

I'd say that this is a bug. Frontend size is guest ABI and
shouldn't be overridden by backend configuration if it's
explicitly set.

-- 
Eduardo