From: Sergey Senozhatsky senozhatsky@chromium.org

mainline inclusion from mainline-v6.1-rc1 commit f635725c3905e755a8c3e2dc8cab7fcd0d38977f 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=...

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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: Sergey Senozhatsky senozhatsky@chromium.org Reviewed-by: Brian Geffon bgeffon@google.com Acked-by: Minchan Kim minchan@kernel.org Cc: Nitin Gupta ngupta@vflare.org Signed-off-by: Andrew Morton akpm@linux-foundation.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 24a5f892d64d..e332b4d55359 100644 --- a/drivers/block/zram/zram_drv.c +++ b/drivers/block/zram/zram_drv.c @@ -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) diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h index b5b94e6b6ec8..eb13d0299f89 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)

/* Only 2 bits are allowed for comp priority index */ #define ZRAM_COMP_PRIORITY_MASK 0x3