Re: [PATCH] dax, pmem: add support for msync

[Christoph Hellwig]

On Tue, Sep 01, 2015 at 09:38:03AM +1000, Dave Chinner wrote:
> On Mon, Aug 31, 2015 at 12:59:44PM -0600, Ross Zwisler wrote:
> > For DAX msync we just need to flush the given range using
> > wb_cache_pmem(), which is now a public part of the PMEM API.
> 
> This is wrong, because it still leaves fsync() broken on dax.
> 
> Flushing dirty data to stable storage is the responsibility of the
> writeback infrastructure, not the VMA/mm infrasrtucture. For non-dax
> configurations, msync defers all that to vfs_fsync_range(), because
> it has to be implemented there for fsync() to work.
> 
> Even for DAX, msync has to call vfs_fsync_range() for the filesystem to commit
> the backing store allocations to stable storage, so there's not
> getting around the fact msync is the wrong place to be flushing
> DAX mappings to persistent storage.

DAX does call ->fsync before and after this patch.  And with all
the recent fixes we take care to ensure data is written though the
cache for everything but mmap-access.  With this patch from Ross
we ensure msync writes back the cache before calling ->fsync so that
the filesystem can then do it's work like converting unwritten extents.

The only downside is that previously on Linux you could always use
fsync as a replaement for msymc, which isn't true anymore for DAX.

But given that we need the virtual address to write back the cache
I can't see how to do this differently given that clwb() needs the
user virtual address to flush the cache.
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