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

[Eduardo Habkost]

On Thu, Oct 20, 2016 at 04:18:20PM +0200, Igor Mammedov wrote:
> On Thu, 20 Oct 2016 11:57:11 -0200
> Eduardo Habkost <address@hidden> wrote:
> 
> > On Thu, Oct 20, 2016 at 03:47:32PM +0200, Igor Mammedov wrote:
> > > On Thu, 20 Oct 2016 21:33:53 +0800
> > > Haozhong Zhang <address@hidden> 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.  
> > > I'd avoid 0 sized backend files and enforce non 0 size value
> > > with exact match to actual file size. i.e. let mgmt side take care of
> > > proper backend file allocation.  
> > 
> > This would break compatibility with existing setups that rely on
> > the ftruncate() behavior.
> 
> Question is what exactly rely on truncate behavior?

hugetlbfs or tmpfs + mempolicy would work only if a sparse file
used (otherwise memory would be already allocated before running
QEMU). That means it is easier to simply not set file size before
running QEMU.

I don't know what's libvirt behavior, but I wouldn't want to
break existing scripts that rely on it.

-- 
Eduardo