linux-kernel - Re: [PATCH] mm/gfp: cast GFP_ZONE_TABLE entries to long to avoid shift overflow

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Message-Id: <20251211120448.817a352350813ae84cb600b3@linux-foundation.org>
Date: Thu, 11 Dec 2025 12:04:48 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Yankai Xu <815559068@...com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/gfp: cast GFP_ZONE_TABLE entries to long to avoid
 shift overflow

On Thu, 11 Dec 2025 18:24:05 +0800 Yankai Xu <815559068@...com> wrote:

> 
> When a new zone is added to enum zone_type in mmzone.h, the existing
> GFP_ZONE_TABLE macro causes a compile-time shift overflow.

Thanks.

Please quote the compiler warning messages and describe how to
reproduce this.  Please include this info in the patch changelog and
maintain it.

> This happens
> because enum zone_type is of type int. The shift is logically
> valid because it has been checked by 16 * GFP_ZONES_SHIFT > BITS_PER_LONG previously.
> 
> Fixed this by casting each value in GFP_ZONE_TABLE to type long.
> 
> ...
>
>  #define GFP_ZONE_TABLE ( \
> -	(ZONE_NORMAL << 0 * GFP_ZONES_SHIFT)				\
> +	((long)ZONE_NORMAL << 0 * GFP_ZONES_SHIFT)			\

It would be nice to find something nicer than this.  Perhaps a type of
something was poorly chosen.