[PATCHv2] zram: do not waste zram_table_entry flags bits

[Sergey Senozhatsky <senozhatsky@chromium.org>]

zram_table_entry::flags stores object size in the lower bits and
zram pageflags in the upper bits. However, for some reason, we
use 24 lower bits, while maximum zram object size is PAGE_SIZE,
which requires PAGE_SHIFT bits (up to 16 on arm64). This wastes
24 - PAGE_SHIFT bits that we can use for additional zram pageflags
instead.

Also add a BUILD_BUG_ON() to alert us should we run out of bits
in zram_table_entry::flags.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 drivers/block/zram/zram_drv.c |  2 ++
 drivers/block/zram/zram_drv.h | 15 +++++++--------
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index f3948abce2f7..07913bcdb5c2 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -2449,6 +2449,8 @@ static int __init zram_init(void)
 {
 	int ret;
 
+	BUILD_BUG_ON(__NR_ZRAM_PAGEFLAGS > BITS_PER_LONG);
+
 	ret = cpuhp_setup_state_multi(CPUHP_ZCOMP_PREPARE, "block/zram:prepare",
 				      zcomp_cpu_up_prepare, zcomp_cpu_dead);
 	if (ret < 0)
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index b4eecef2a11f..2b50f0521bd3 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -30,16 +30,15 @@
 
 
 /*
- * The lower ZRAM_FLAG_SHIFT bits of table.flags is for
- * object size (excluding header), the higher bits is for
- * zram_pageflags.
+ * ZRAM is mainly used for memory efficiency so we want to keep memory
+ * footprint small and thus squeeze size and zram pageflags into a flags
+ * member. The lower ZRAM_FLAG_SHIFT bits is for object size (excluding
+ * header), which cannot be larger than PAGE_SIZE (requiring PAGE_SHIFT
+ * bits), the higher bits are for zram_pageflags.
  *
- * zram is mainly used for memory efficiency so we want to keep memory
- * footprint small so we can squeeze size and flags into a field.
- * The lower ZRAM_FLAG_SHIFT bits is for object size (excluding header),
- * the higher bits is for zram_pageflags.
+ * We use BUILD_BUG_ON() to make sure that zram pageflags don't overflow.
  */
-#define ZRAM_FLAG_SHIFT 24
+#define ZRAM_FLAG_SHIFT (PAGE_SHIFT + 1)
 
 /* Flags for zram pages (table[page_no].flags) */
 enum zram_pageflags {
-- 
2.37.2.789.g6183377224-goog