The next patch embeds struct zs_handle into zram and zswap entries,
and a handle is considered allocated when its obj is not 0.  But obj
can be 0: object 0 of a zspage whose first page is PFN 0 encodes to
0, and some platforms do give PFN 0 to the page allocator.

Store obj_idx + 1 in obj, so that a valid obj is never 0, and adjust
ZS_MIN_ALLOC_SIZE so that the largest index still fits.  Unlike a
separate tag bit, this doesn't take any bits from obj.  zsmalloc did
the same before handles moved to handle_cachep, see commit 67296874eb1c
("staging: zsmalloc: Ensure handle is never 0 on success").

Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 mm/zsmalloc.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 2385c1951ca7..9be538d0be22 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -61,7 +61,8 @@
  * Object location (<PFN>, <obj_idx>) is encoded as
  * a single (unsigned long) handle value.
  *
- * Note that object index <obj_idx> starts from 0.
+ * Note that object index <obj_idx> starts from 0, and is encoded as
+ * <obj_idx> + 1 so that a valid encoded obj value is never 0.
  *
  * This is made more complicated by various memory models and PAE.
  */
@@ -169,9 +170,6 @@
 static_assert(ZS_OBJ_IDX_BITS >= ZS_PAGES_PER_ZSPAGE_BITS,
 	      "zsmalloc: ZS_MIN_ALLOC_SIZE would exceed ZS_MAX_ALLOC_SIZE");
 
-/* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
-#define ZS_MIN_ALLOC_SIZE \
-	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> ZS_OBJ_IDX_BITS))
 /* each chunk includes extra space to keep handle */
 #define ZS_MAX_ALLOC_SIZE	PAGE_SIZE
 
@@ -189,6 +187,11 @@ static_assert(ZS_OBJ_IDX_BITS >= ZS_PAGES_PER_ZSPAGE_BITS,
  *  (reason above)
  */
 #define ZS_SIZE_CLASS_DELTA	(PAGE_SIZE >> CLASS_BITS)
+#define _ZS_MIN_ALLOC_RAW \
+	DIV_ROUND_UP(ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT, ZS_OBJ_IDX_MASK)
+#define ZS_MIN_ALLOC_SIZE \
+	MIN(ZS_MAX_ALLOC_SIZE, \
+	    MAX(32UL, ALIGN(_ZS_MIN_ALLOC_RAW, ZS_ALIGN)))
 #define ZS_SIZE_CLASSES	(DIV_ROUND_UP(ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE, \
 				      ZS_SIZE_CLASS_DELTA) + 1)
 
@@ -794,8 +797,10 @@ static struct zpdesc *get_next_zpdesc(struct zpdesc *zpdesc)
 static void obj_to_location(unsigned long obj, struct zpdesc **zpdesc,
 				unsigned int *obj_idx)
 {
+	VM_WARN_ON_ONCE(!(obj & ZS_OBJ_IDX_MASK));
+
 	*zpdesc = pfn_zpdesc(obj >> ZS_OBJ_PFN_SHIFT);
-	*obj_idx = (obj & ZS_OBJ_IDX_MASK);
+	*obj_idx = (obj & ZS_OBJ_IDX_MASK) - 1;
 }
 
 static void obj_to_zpdesc(unsigned long obj, struct zpdesc **zpdesc)
@@ -814,9 +819,11 @@ static unsigned long location_to_obj(struct zpdesc *zpdesc, unsigned int obj_idx
 {
 	unsigned long obj;
 
+	VM_WARN_ON_ONCE(obj_idx + 1 > ZS_OBJ_IDX_MASK);
+
 	obj  = zpdesc_pfn(zpdesc) << ZS_OBJ_PFN_SHIFT;
 	obj |= (unsigned long)(class_idx & ZS_OBJ_CLASS_MASK) << ZS_OBJ_IDX_BITS;
-	obj |= obj_idx & ZS_OBJ_IDX_MASK;
+	obj |= (obj_idx + 1) & ZS_OBJ_IDX_MASK;
 
 	return obj;
 }
-- 
2.56.0.542.g3ef89b7020-goog