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


From: Eduardo Habkost
Subject: Re: [Qemu-devel] [PATCH] hostmem-file: add a property 'notrunc' to avoid data corruption
Date: Thu, 20 Oct 2016 11:57:11 -0200
User-agent: Mutt/1.7.0 (2016-08-17)

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.

-- 
Eduardo



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