Re: [Qemu-devel] [PATCH v14 0/2] support MAP_SYNC for memory-backend-file

[Wei Yang]

On Mon, Apr 22, 2019 at 03:22:55PM -0300, Eduardo Habkost wrote:
>On Mon, Apr 22, 2019 at 08:34:51AM -0400, Michael S. Tsirkin wrote:
>> On Mon, Apr 22, 2019 at 08:48:47AM +0800, Wei Yang 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.
>> > 
>> > We will pass MAP_SYNC to mmap(2); if MAP_SYNC is supported and
>> > 'share=on' & 'pmem=on'. 
>> > Or QEMU will not pass this flag to mmap(2)
>> 
>> OK this is in a good shape. As we are in freeze anyway,
>> there's still a bit more time to polish it. I have a couple of
>> suggestions:
>> 
>> - squash docs in same patch with code, no need for two patches
>> - mmap errors are not silently ignored as the doc says,
>>   a warning is produced
>
>Note that a warning is produced only if both share=on and pmem=on
>is specified.  If using pmem=on without share=on, no warning is
>printed at all.
>
>I agree we could squash the docs in the same patch, but I don't
>want to prevent the code from being merged and require v15 to be
>sent just because we are still polishing the documentation.
>
>If there are no objections, I plan to apply this version of the
>series including the following fixup (just removing the word
>"silently"), and I suggest further improvements to be sent as
>follow up patches.
>

If my understanding is correct, the following up patch is:

"send the warnings to an errp object and not stderr"

-- 
Wei Yang
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