Re: [PATCHv2] zram: do not waste zram_table_entry flags bits

[Brian Geffon <bgeffon@google.com>]

On Mon, Sep 12, 2022 at 11:29 AM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> 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);

Should this be >= 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
>