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

[Igor Mammedov]

On Thu, 20 Oct 2016 14:56:58 -0200
Eduardo Habkost <address@hidden> wrote:

> On Thu, Oct 20, 2016 at 05:35:38PM +0200, Igor Mammedov wrote:
> > On Thu, 20 Oct 2016 12:47:34 -0200
> > Eduardo Habkost <address@hidden> wrote:
> >   
> > > On Thu, Oct 20, 2016 at 04:15:21PM +0200, Igor Mammedov wrote:  
> > > > On Thu, 20 Oct 2016 11:56:10 -0200
> > > > Eduardo Habkost <address@hidden> wrote:
> > > >     
> > > > > On Thu, Oct 20, 2016 at 03:42:15PM +0200, Igor Mammedov wrote:    
> > > > > > On Thu, 20 Oct 2016 21:11:38 +0800
> > > > > > Haozhong Zhang <address@hidden> wrote:
> > > > > >       
> > > > > > > On 10/20/16 14:34 +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 don't know how this can be implemented in QEMU. Specially, how 
> > > > > > > does
> > > > > > > the memory-backend-file know it's used for vNVDIMM, so that it can
> > > > > > > skip the 'size' property?      
> > > > > > Does memory-backend-file needs to know that it's used by NVDIMM?
> > > > > > Looking at nvdimm_realize it doesn't as it's assumes 
> > > > > >   hostemem_size == pmem_size + label_size
> > > > > > 
> > > > > > I'd make hostmem_file.size optional and take size from file
> > > > > > and if 'size' is specified explictly require it to mach file size.
> > > > > > It's generic and has nothing to do with nvdimm.      
> > > > > 
> > > > > We can take size from file, or take size from the
> > > > > host_memory_backend_get_memory() callers.
> > > > > 
> > > > > Enumerating all sizes that QEMU can use as input:
> > > > > 
> > > > > A) Backend file size
> > > > > B) memory backend "size" option
> > > > > C) frontend-provided size (-numa size, -m, or pc-dimm "size"
> > > > >     property)    
> > > > -numa size affect only anon memory not backend backed one, for
> > > > backend baked memory we use memdev where size comes from backend
> > > > 
> > > > pc-dimm.size is readonly and isn't supposed to influence backend.size
> > > > 
> > > > I'd drop C option    
> > > 
> > > If C is not present, it should be, as it affects the guest ABI
> > > (and the ABI must never depend on the host you are running or
> > > backend configuration, only on the frontend configuration).  
> > I've meant that C should not affect behavior of backend.
> >   
> > > If we are dropping -numa size in favor of the
> > > memory-backend-provided size, that's a bug.  
> > -numa size is not applicable here as it's not using backends,
> > when backends are used it's -numa memdev instead in which case
> >    numa_info[nodenr].node_mem = object_property_get_int(o, "size", NULL);  
> 
> OK. My suggestion is to change this to not ignore the size
> option, but to validate it and/or do whatever necessary to get a
> MemoryRegion of the right size (your suggestion below to split
> the memory region would work too).
> 
> In other words, this should work:
> 
>   $ qemu -object memory-backend-file,id=mem0,mem-path=/tmp/mempath,size=2G \
>          -numa node,size=2G,memdev=mem0 -m 2G
> 
> this must error out:
> 
>   $ qemu -object memory-backend-file,id=mem0,mem-path=/tmp/mempath,size=2G \
>          -numa node,size=4G,memdev=mem0 -m 2G
> 
> and this should either error out, or result in a 1GB NUMA node
> (but never in a 2G NUMA node):
> 
>   $ qemu -object memory-backend-file,id=mem0,mem-path=/tmp/mempath,size=2G \
>          -numa node,size=1G,memdev=mem0 -m 1G
Looks like a bit of over-engineering and mixing together backend with frontend,
I don't see a compelling reason to support
  -numa size=X,memdev=foo
as just memdev=foo is sufficient

so I'd rather error out if user adds 'size' to -numa memdev=foo

[...]

> Probably there's only one case where behavior would be different
> than what I was thinking. I would like to be possible to specify
> only C (frontend size), and omit both A and B. For example:
> 
>   $ mkdir /tmp/mempath
>   $ qemu -object memory-backend-file,id=mem0,mem-path=/tmp/mempath \
>          -numa node,size=1G,memdev=mem0 -m 1G
> 
> I would like this command to create a 1G file inside /tmp/mempath
> automatically, without the need for an explicit size=1G argument
> to memory-backend-file.
This is still C where frontend manages allocation of backend
(i.e. changes backend's 'size' property) vs being just consumer
of whatever backend provides.
Purpose of backend is to create complete/initialized hostside
object regardless of which frontend uses it.
I see all properties of backend (including sizes) as belonging
to hostside which shouldn't be influenced by whatever frontend
it's used. And these properties are specified/managed only by mgmt
side explicitly when creating a backend (either via CLI or monitor/QMP).

In your example I'd error out with:
  "memory-backend-file: requires 'size' to create file in /tmp/mempath"