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 <20210924125422.358374d83cdb95db055a4467@linux-foundation.org>
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From:   Brian Geffon <bgeffon@google.com>
Date:   Fri, 24 Sep 2021 16:15:26 -0400
Message-ID: <CADyq12xAc4Y5P4n69oahPNs6M55cUb6=7Nku=J5iEOJVMBOPTQ@mail.gmail.com>
Subject: Re: [PATCH v5] zram: Introduce an aged idle interface
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Minchan Kim <minchan@kernel.org>, Nitin Gupta <ngupta@vflare.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Jonathan Corbet <corbet@lwn.net>,
        LKML <linux-kernel@vger.kernel.org>, linux-doc@vger.kernel.org,
        linux-block@vger.kernel.org,
        Suleiman Souhlal <suleiman@google.com>,
        Jesse Barnes <jsbarnes@google.com>
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> Also this?
>
> --- a/drivers/block/zram/zram_drv.c~zram-introduce-an-aged-idle-interface-v5-fix
> +++ a/drivers/block/zram/zram_drv.c
> @@ -309,9 +309,8 @@ static void mark_idle(struct zram *zram,
>                 zram_slot_lock(zram, index);
>                 if (zram_allocated(zram, index) &&
>                                 !zram_test_flag(zram, index, ZRAM_UNDER_WB)) {
> -#ifdef CONFIG_ZRAM_MEMORY_TRACKING
> +                       if (IS_ENABLED(CONFIG_ZRAM_MEMORY_TRACKING))
>                                 is_idle = (!cutoff || ktime_after(cutoff, zram->table[index].ac_time));
> -#endif
>                         if (is_idle)
>                                 zram_set_flag(zram, index, ZRAM_IDLE);
>                 }
> _
>

Hi Andrew,
As written that patch won't compile when
CONFIG_ZRAM_MEMORY_TRACKING=n, my guess is that the compiler pass that
removes the dead branch only happens after it attempts to compile the
branch itself. So it appears that even though IS_ENABLED(..) always
evaluates to 0, the compile will fail because table[index].ac_time
does not exist. You should get an error like this:

drivers/block/zram/zram_drv.c:314:57: error: no member named 'ac_time'
in 'struct zram_table_entry'
                                                             (!cutoff
|| ktime_after(cutoff, zram->table[index].ac_time)))

Brian