Commit 84f941ba authored by Sergey Senozhatsky's avatar Sergey Senozhatsky Committed by Jinjiang Tu
Browse files

zram: do not waste zram_table_entry flags bits 

mainline inclusion
from mainline-v6.1-rc1
commit f635725c
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7TWVA
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f635725c3905e755a8c3e2dc8cab7fcd0d38977f

-------------------------------------------

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.

Link: https://lkml.kernel.org/r/20220912152744.527438-1-senozhatsky@chromium.org


Signed-off-by: default avatarSergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: default avatarBrian Geffon <bgeffon@google.com>
Acked-by: default avatarMinchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 518d7451

drivers/block/zram/zram_drv.c

+2 −0
Original line number Diff line number Diff line
@@ -2591,6 +2591,8 @@ 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)

drivers/block/zram/zram_drv.h

+7 −8
Original line number Diff line number Diff line
@@ -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)



/* Only 2 bits are allowed for comp priority index */

#define ZRAM_COMP_PRIORITY_MASK	0x3