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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Sep 02, 2015 at 01:04:01PM -0600, Ross Zwisler wrote:
> On Tue, Sep 01, 2015 at 03:18:41PM +0300, Boaz Harrosh wrote:
> > So the approach we took was a bit different to exactly solve these
> > problem, and to also not over flush too much. here is what we did.
> > 
> > * At vm_operations_struct we also override the .close vector (say call it dax_vm_close)
> > 
> > * At dax_vm_close() on writable files call ->fsync(,vma->vm_start, vma->vm_end,)
> >   (We have an inode flag if the file was actually dirtied, but even if not, that will
> >    not be that bad, so a file was opened for write, mmapped, but actually never
> >    modified. Not a lot of these, and the do nothing cl_flushing is very fast)
> > 
> > * At ->fsync() do the actual cl_flush for all cases but only iff
> > 	if (mapping_mapped(inode->i_mapping) == 0)
> > 		return 0;
> > 
> >   This is because data written not through mmap is already persistent and we
> >   do not need the cl_flushing
> > 
> > Apps expect all these to work:
> > 1. open mmap m-write msync ... close
> > 2. open mmap m-write fsync ... close
> > 3. open mmap m-write unmap ... fsync close
> > 
> > 4. open mmap m-write sync ...
> 
> So basically you made close have an implicit fsync?  What about the flow that
> looks like this:
> 
> 5. open mmap close m-write
> 
> This guy definitely needs an msync/fsync at the end to make sure that the
> m-write becomes durable.  

We can sync on pte_dirty() during zap_page_range(): it's practically free,
since we page walk anyway.

With this approach it probably makes sense to come back to page walk on
msync() side too to be consistent wrt pte_dirty() meaning.

> Also, the CLOSE(2) man page specifically says that a flush does not occur at
> close:
> 	A successful close does not guarantee that the data has been
> 	successfully  saved  to  disk,  as  the  kernel defers  writes.   It
> 	is not common for a filesystem to flush the buffers when the stream is
> 	closed.  If you need to be sure that the data is physically stored,
> 	use fsync(2).  (It will depend on the disk  hardware  at this point.)
> 
> I don't think that adding an implicit fsync to close is the right solution -
> we just need to get msync and fsync correctly working.

I doesn't mean we can't sync if we can do without noticible performance
degradation.

-- 
 Kirill A. Shutemov

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