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Date:   Mon, 12 Sep 2022 23:51:42 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Brian Geffon <bgeffon@google.com>, Minchan Kim <minchan@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Nitin Gupta <ngupta@vflare.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>
Subject: Re: [PATCH] zram: do not waste zram_table_entry flags bits
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References: <20220912053755.156704-1-senozhatsky@chromium.org>
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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);
+
        ret = cpuhp_setup_state_multi(CPUHP_ZCOMP_PREPARE, "block/zram:prepare",
                                      zcomp_cpu_up_prepare, zcomp_cpu_dead);
        if (ret < 0)