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

[Michael S. Tsirkin]

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.
> 
> diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> index bcd1456e72..b531cacd35 100644
> --- a/docs/nvdimm.txt
> +++ b/docs/nvdimm.txt
> @@ -159,8 +159,8 @@ If these conditions are not satisfied i.e. if either 
> 'pmem' or 'share'
>  are not set, if the backend file does not support DAX or if MAP_SYNC
>  is not supported by the host kernel, write persistence is not
>  guaranteed after a system crash. For compatibility reasons, these
> -conditions are silently ignored if not satisfied. Currently, no way
> -is provided to test for them.
> +conditions are ignored if not satisfied. Currently, no way is
> +provided to test for them.
>  For more details, please reference mmap(2) man page:
>  http://man7.org/linux/man-pages/man2/mmap.2.html.

with the two being squashed, and above fix:

Reviewed-by: Michael S. Tsirkin <address@hidden>



> -- 
> Eduardo