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

[Igor Mammedov]

On Thu, 20 Oct 2016 13:15:31 -0200
Eduardo Habkost <address@hidden> wrote:

> 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.
frontend.size is always <= backend.size

allocation specified when backend is created (-object/object_add)
and front end size if needed/used is <= backend size

so far code followed this design.