[Virtio-fs] data integrity in dax mode

[Liu Bo]

On Tue, Jul 02, 2019 at 01:43:58PM -0400, Vivek Goyal wrote:
> On Mon, Jul 01, 2019 at 10:59:38PM -0700, Liu Bo wrote:
> > Hi,
> > 
> > I have a concern about data integrity in dax mode.
> > 
> > For now virtio_fs_copy_from_iter() (i.e. the write direction) only
> > does copy_from_iter(), which doesn't enforce a clean_cache_range or
> > CLWB.
> 
> This is an interesting question and I had been wondring about it when
> I wrote the code.
> 
> I could not justify the need of calling "clwb". Whatever we write will
> be in host page cache (assuming dir is not pmem backed). And when app
> in guest does fsync/msync, only then there are any guarantees that
> data is persistent.
> 
> IOW, we are not offering pmem programming interface where msync can 
> be avoided by applications. Apps still need to call in fsync/msync
> to ensure that data has been saved to persistent storage. And if
> that happens, I am not sure why clwb needs to be called.
>

I agree. As long as fuse_dax_writepages is calling
dax_writeback_mapping_range, data integrity can be ensured via
fsync/msync.

> > And even worse, virtio-fs doesn't have a dax_writepages like,
> > which can calls dax_writeback_mapping_range().
> > 
> > Correct me if I'm wrong, but did virtio-fs solve it in some way?
> 
> We define address space operations for dax.
> 
> static const struct address_space_operations fuse_dax_file_aops  = {
>         .writepages     = fuse_dax_writepages,
>         .direct_IO      = noop_direct_IO,
>         .set_page_dirty = noop_set_page_dirty,
>         .invalidatepage = noop_invalidatepage,
> };
> 
> And fuse_dax_writepages() will call dax_writeback_mapping_range().

Good point, somehow I missed this part, now we're good.

Thanks for the clarification.

thanks,
-liubo