On Wed, Jan 05, 2011 at 12:17:27PM -0500, Trond Myklebust wrote:

We should already be flushing the kernel direct mapping after writing by
means of the calls to flush_dcache_page() in xdr_partial_copy_from_skb()
and all the helpers in net/sunrpc/xdr.c.

Hmm, we're getting into the realms of what flush_dcache_page() is supposed
to do and what it's not supposed to do.

Is this page an associated with a mapping (iow, page_mapping(page) is non-
NULL)?  If not, flush_dcache_page() won't do anything, and from my
understanding, its flush_anon_page() which you want to be using there
instead.

The only new thing is the read access through the virtual address
mapping. That mapping is created outside the loop in
nfs_readdir_xdr_to_array(), which is why I'm thinking we do need the
invalidate_kernel_vmap_range(): we're essentially doing a series of
writes through the kernel direct mapping (i.e. readdir RPC calls), then
reading the results through the virtual mapping.

i.e. we're doing

ptr = vm_map_ram(lowmem_pages);
while (need_more_data) {

for (i = 0; i < npages; i++) {
addr = kmap_atomic(lowmem_page[i]);
*addr = rpc_stuff;
flush_dcache_page(lowmem_page[i]);
kunmap_atomic(lowmem_page[i]);
}

invalidate_kernel_vmap_range(ptr); // Needed here?

Yes, you're going to need some cache maintainence in there to make it work,
because accessing 'ptr' will load that data into the cache, and that won't
be updated by the writes via kmap_atomic().

Provided you don't write to ptr, then using invalidate_kernel_vmap_range()
will be safe.