linux-kernel - Re: [PATCHv2] zram: do not waste zram_table

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Message-ID: <YyNUlEGwooXzpF9Z@google.com>
Date:   Thu, 15 Sep 2022 16:36:36 +0000
From:   Minchan Kim <minchan@...nel.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Nitin Gupta <ngupta@...are.org>,
        Brian Geffon <bgeffon@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv2] zram: do not waste zram_table_entry flags bits

On Tue, Sep 13, 2022 at 12:27:44AM +0900, Sergey Senozhatsky 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@...omium.org>
Acked-by: Minchan Kim <minchan@...nel.org>