Re: [PATCH] dax: Allow block size > PAGE_SIZE

[Dan Williams <dan.j.williams@intel.com>]

Asahi Lina wrote:
[..]
> I don't think that's how it actually works, at least on arm64.
> arch_wb_cache_pmem() calls dcache_clean_pop() which is either dc cvap or
> dc cvac. Those are trapped by HCR_EL2<TPC>, and that is never set by KVM.
> 
> There was some discussion of this here:
> https://lore.kernel.org/all/20190702055937.3ffpwph7anvohmxu@US-160370MP2.local/
> 
> But I'm not sure that all really made sense then.
> 
> msync() and fsync() should already provide persistence. Those end up
> calling vfs_fsync_range(), which becomes a FUSE fsync(), which fsyncs
> (or fdatasyncs) the whole file. What I'm not so sure is whether there
> are any other codepaths that also need to provide those guarantees which
> *don't* end up calling fsync on the VFS. For example, the manpages kind
> of imply munmap() syncs, though as far as I can tell that's not actually
> the case. If there are missing sync paths, then I think those might just
> be broken right now...

IIRC, from the pmem persistence dicussions, if userspace fails to call
*sync then there is no obligation to flush on munmap() or close(). Some
filesystems layer on those guarantees, but the behavior is
implementation specific.