linux-kernel - Re: [PATCH] mm/zsmalloc: include sparsemem.h for MAX_PHYSMEM

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Date:   Sat, 7 Nov 2020 16:56:45 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Stefan Agner <stefan@...er.ch>
Cc:     minchan@...nel.org, ngupta@...are.org,
        sergey.senozhatsky.work@...il.com, sjenning@...ux.vnet.ibm.com,
        gregkh@...uxfoundation.org, arnd@...db.de, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/zsmalloc: include sparsemem.h for MAX_PHYSMEM_BITS

On Sat,  7 Nov 2020 16:22:06 +0100 Stefan Agner <stefan@...er.ch> wrote:

> Most architectures define MAX_PHYSMEM_BITS in asm/sparsemem.h and don't
> include it in asm/pgtable.h. Include asm/sparsemem.h directly to get
> the MAX_PHYSMEM_BITS define on all architectures.
> 
> This fixes a crash when accessing zram on 32-bit ARM platform with LPAE and
> more than 4GB of memory:
>   Unable to handle kernel NULL pointer dereference at virtual address 00000000

Mysterious.  Presumably without this include, some compilation unit is
picking up the wrong value of MAX_PHYSMEM_BITS?  But I couldn't
actually see where/how this occurs.  Can you please explain further?