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Re: [Qemu-devel] [PATCH V7 0/6] nvdimm: support MAP_SYNC for memory-back

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH V7 0/6] nvdimm: support MAP_SYNC for memory-backend-file
Date: Tue, 18 Dec 2018 09:18:02 +0000
User-agent: Mutt/1.10.1 (2018-07-13)

On Tue, Dec 18, 2018 at 04:16:44PM +0800, Zhang Yi wrote:
> Linux 4.15 introduces a new mmap flag MAP_SYNC, which can be used to
> guarantee the write persistence to mmap'ed files supporting DAX (e.g.,
> files on ext4/xfs file system mounted with '-o dax').
> A description of MAP_SYNC and MAP_SHARED_VALIDATE can be found at
>     https://patchwork.kernel.org/patch/10028151/
> In order to make sure that the file metadata is in sync after a fault 
> while we are writing a shared DAX supporting backend files, this
> patch-set enables QEMU to use MAP_SYNC flag for memory-backend-dax-file.
> As the DAX vs DMA truncated issue was solved, we refined the code and
> send out this feature for the v5 version.
> A new auto on/off option 'sync' is added to memory-backend-file:
>  - on:  try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
>         'share=off' or 'pmem=off', QEMU will abort
>  - off: never pass MAP_SYNC to mmap(2)
>  - auto (default): if MAP_SYNC is supported and 'share=on' 'pmem=on', work as 
> if
>         'sync=on'; otherwise, work as if 'sync=off'

I took a quick look and didn't see human-readable errors when pmem=off.
Is it possible to report it instead of aborting?


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