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Date: Thu, 20 Oct 2016 15:47:32 +0200
From: Igor Mammedov <imammedo@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] hostmem-file: add a property 'notrunc' to
 avoid data corruption
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To: Haozhong Zhang <haozhong.zhang@intel.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>, qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Richard Henderson <rth@twiddle.net>, Xiao
	Guangrong <guangrong.xiao@linux.intel.com>, kwolf@redhat.com, mreitz@redhat.com

On Thu, 20 Oct 2016 21:33:53 +0800
Haozhong Zhang <haozhong.zhang@intel.com> 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 <haozhong.zhang@intel.com> 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.

> 
> >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?
> 
> Thanks,
> Haozhong