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From: | Haozhong Zhang |
Subject: | Re: [Qemu-devel] [PATCH] hostmem-file: add a property 'notrunc' to avoid data corruption |
Date: | Thu, 20 Oct 2016 21:33:53 +0800 |
User-agent: | NeoMutt/20160827 (1.7.0) |
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? Thanks, Haozhong
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