staging: zcache: handle zcache_[eph|pers]_zpages for zero-filled page

Increment/decrement zcache_[eph|pers]_zpages for zero-filled pages,
the main point of the counters for zpages and pageframes is to be
able to calculate density == zpages/pageframes. A zero-filled page
becomes a zpage that "compresses" to zero bytes and, as a result,
requires zero pageframes for storage. So the zpages counter should
be increased but the pageframes counter should not.

[Dan Magenheimer <dan.magenheimer@oracle.com>: patch description]
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zcache/zcache-main.c

@@ -648,6 +648,8 @@ static int zcache_pampd_get_data_and_free(char *data, size_t *sizep, bool raw,
 	if (pampd == (void *)ZERO_FILLED) {
 		handle_zero_filled_page(data);
 		zero_filled = true;
+		zsize = 0;
+		zpages = 1;
 		if (!raw)
 			*sizep = PAGE_SIZE;
 		goto zero_fill;
@@ -696,8 +698,11 @@ static void zcache_pampd_free(void *pampd, struct tmem_pool *pool,
 
 	BUG_ON(preemptible());
 
-	if (pampd == (void *)ZERO_FILLED)
+	if (pampd == (void *)ZERO_FILLED) {
 		zero_filled = true;
+		zsize = 0;
+		zpages = 1;
+	}
 
 	if (pampd_is_remote(pampd) && !zero_filled) {
 		BUG_ON(!ramster_enabled);