he completion function will be given
     an error, and will also be passed contextual information to allow the
     netfs to fall back to querying the server for the absent pages.

     For each successful read, the page completion function will also be
     called.

     Any pages subsequently tracked by the cache will have PG_fscache set upon
     them on return.  fscache_uncache_page() must be called for such pages.

     If supplied by the netfs, the mark_pages_cached() cookie op will be
     invoked for any pages now tracked.

 (*) fscache_alloc_page().

     Allocate internal metadata to track a netfs page and reserve disk space.

     This will return -ENOMEM on memory allocation error, -ERESTARTSYS on
     signal, -ENOBUFS if the object isn't cached, or there isn't enough space
     in the cache, or 0 if successful.

     Any pages subsequently tracked by the cache will have PG_fscache set upon
     them on return.  fscache_uncache_page() must be called for such pages.

     If supplied by the netfs, the mark_pages_cached() cookie op will be
     invoked for any pages now tracked.

 (*) fscache_write_page().

     Request data be stored to disk.  This may only be called on pages that
     have been read or alloc'd by the above three functions and have not yet
     been uncached.

     This will return -ENOMEM on memory allocation error, -ERESTARTSYS on
     signal, -ENOBUFS if the object isn't cached, or there isn't immediately
     enough space in the cache, or 0 if successful.

     On a successful return, this operation will have queued the page for
     asynchronous writing to the cache.  The page will be returned with
     PG_fscache_write set until the write completes one way or another.  The
     caller will not be notified if the write fails due to an I/O error.  If
     that happens, the object will become available and all pending writes will
     be aborted.

     Note that the cache may batch up page writes, and so it may take a while
     to get around to writing them out.

     The caller must assume that until PG_fscache_write is cleared the page is
     use by the cache.  Any changes made to the page may be reflected on disk.
     The page may even be under DMA.

 (*) fscache_uncache_page().

     Indicate that the cache should stop tracking a page previously read or
     alloc'd from the cache.  If the page was alloc'd only, but unwritten, it
     will not appear on disk.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>
3[