virtiofs: dax: remove ->writepages() callback

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.

Since dax_writeback_mapping_range() checks that the inode block size is
equal to PAGE_SIZE, this fixes a spurious WARN when virtiofs is used
with a mismatched guest PAGE_SIZE and virtiofs backing FS block size
(this happens, for example, when it's a tmpfs and the host and guest
have a different PAGE_SIZE). FUSE DAX does not require any particular FS
block size, since it always performs DAX mappings in aligned 2MiB
blocks.

See discussion in [1].

[1] https://lore.kernel.org/lkml/20241101-dax-page-size-v1-1-eedbd0c6b08f@asahilina.net/T/#u

[SzM: remove the empty callback]

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Asahi Lina <lina@asahilina.net>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: asahilina

fs/fuse/dax.c

@@ -774,16 +774,6 @@ ssize_t fuse_dax_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	return ret;
 }
 
-static int fuse_dax_writepages(struct address_space *mapping,
-			       struct writeback_control *wbc)
-{
-
-	struct inode *inode = mapping->host;
-	struct fuse_conn *fc = get_fuse_conn(inode);
-
-	return dax_writeback_mapping_range(mapping, fc->dax->dev, wbc);
-}
-
 static vm_fault_t __fuse_dax_fault(struct vm_fault *vmf, unsigned int order,
 		bool write)
 {
@@ -1323,7 +1313,6 @@ bool fuse_dax_inode_alloc(struct super_block *sb, struct fuse_inode *fi)
 }
 
 static const struct address_space_operations fuse_dax_file_aops  = {
-	.writepages	= fuse_dax_writepages,
 	.direct_IO	= noop_direct_IO,
 	.dirty_folio	= noop_dirty_folio,
 };