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

[Brian Geffon <bgeffon@google.com>]

On Mon, Sep 12, 2022 at 10:51 AM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> On (22/09/12 23:37), Sergey Senozhatsky wrote:
> > > > -#define ZRAM_FLAG_SHIFT 24
> > > > +#define ZRAM_FLAG_SHIFT (PAGE_SHIFT + 1)
> > >
> > > Why not just hard code 16 with an explanation that it cannot be
> > > increased further using the analysis you did in the other thread? It's
> > > going to be tricky to reason about how many free flag bits actually
> > > remain with PAGE_SHIFT across all architectures, especially given we
> > > have no architecture specific flags.
> >
> > Well, zram should not make any assumptions on arch code. How do
> > we know that PAGE_SHIFT 16 is the max value we will ever have?
> > Some arch can come around someday and use PAGE_SHIFT say, 18,
> > and we won't be aware of it (using hardcoded value of 16) until
> > someone hits a really hard to debug problem in zram.
>
> And I'd probably also add something like this, to keep us alert should
> we run out of bits in the future:
>
> ---
>
> 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);
> +

Thanks Sergey, yes, with the BUILD_BUG_ON I think using PAGE_SHIFT is
fine, my concern was primarily that a flag could overwrite a bit of
the size field, a BUILD_BUG_ON addresses that.

>         ret = cpuhp_setup_state_multi(CPUHP_ZCOMP_PREPARE, "block/zram:prepare",
>                                       zcomp_cpu_up_prepare, zcomp_cpu_dead);
>         if (ret < 0)

Thanks,
Brian