zram: Fixes for lookup_swap_cache()

Fix the following three issues:
(1) swap cache missing page tree offset
    The radix or xarray start at an offset inside struct address_space.
(2) swap cache entries are pointer to struct page
    The entries in radix, xarray (swap cache) are address to struct page.
(3) exclude shadow entries from swap cache lookup
    radix or xarray can contain shadow entries from previous page
    entries. These should be ignored when looking for a page pointer.

Without the patch,
- lookup_swap_cache() returns NULL since do_xarray() call returns FALSE,
- in try_zram_decompress(), since 'entry' is NULL, page is filled with 0,
    if (!entry || (flags & ZRAM_FLAG_SAME_BIT)) {
and pages in swap cache will be seen to be a 'zero' page.

Signed-off-by: Johan Erlandsson <[email protected]>
Signed-off-by: Kazuhito Hagio <[email protected]>

diskdump.c

@@ -27,6 +27,7 @@
 #include "diskdump.h"
 #include "xen_dom0.h"
 #include "vmcore.h"
+#include "maple_tree.h"
 
 #define BITMAP_SECT_LEN	4096
 
@@ -2877,11 +2878,16 @@ zram_object_addr(ulong pool, ulong handle, unsigned char *zram_buf)
 	return zram_buf;
 }
 
+static inline bool radix_tree_exceptional_entry(ulong entry)
+{
+	return entry & RADIX_TREE_EXCEPTIONAL_ENTRY;
+}
+
 static unsigned char *
 lookup_swap_cache(ulonglong pte_val, unsigned char *zram_buf)
 {
 	ulonglong swp_offset;
-	ulong swp_type, swp_space, page;
+	ulong swp_type, swp_space;
 	struct list_pair lp;
 	physaddr_t paddr;
 	static int is_xarray = -1;
@@ -2907,10 +2913,13 @@ lookup_swap_cache(ulonglong pte_val, unsigned char *zram_buf)
 	swp_space += (swp_offset >> SWAP_ADDRESS_SPACE_SHIFT) * SIZE(address_space);
 
 	lp.index = swp_offset;
-	if ((is_xarray ? do_xarray : do_radix_tree)(swp_space, RADIX_TREE_SEARCH, &lp)) {
-		readmem((ulong)lp.value, KVADDR, &page, sizeof(void *),
-				"swap_cache page", FAULT_ON_ERROR);
-		if (!is_page_ptr(page, &paddr)) {
+	if ((is_xarray ? do_xarray : do_radix_tree)
+		(swp_space+OFFSET(address_space_page_tree), RADIX_TREE_SEARCH, &lp)) {
+		if ((is_xarray ? xa_is_value : radix_tree_exceptional_entry)((ulong)lp.value)) {
+			/* ignore shadow values */
+			return NULL;
+		}
+		if (!is_page_ptr((ulong)lp.value, &paddr)) {
 			error(WARNING, "radix page: %lx: not a page pointer\n", lp.value);
 			return NULL;
 		}