Re: [PATCH] fuse: dax: No-op writepages callback

[Miklos Szeredi]

On Tue, 12 Nov 2024 at 20:55, Asahi Lina <lina@xxxxxxxxxxxxx> wrote:
>
> When using FUSE DAX with virtiofs, cache coherency is managed by the
> host. Disk persistence is handled via fsync() and friends, which are
> passed directly via the FUSE layer to the host. Therefore, there's no
> need to do dax_writeback_mapping_range(). All that ends up doing is a
> cache flush operation, which is not caught by KVM and doesn't do much,
> since the host and guest are already cache-coherent.

The conclusion seems convincing.  But adding Vivek, who originally
added this in commit 9483e7d5809a ("virtiofs: define dax address space
operations").

What I'm not clearly seeing is how virtually aliased CPU caches
interact with this.  In mm/filemap.c I see the flush_dcache_folio()
calls which deal with the kernel mapping of a page being in a
different cacheline as the user mapping.  How does that work in the
virt environment?

Also I suggest to remove the writepages callback instead of leaving it
as a no-op.

Thanks,
Miklos