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Date:   Sun, 08 Nov 2020 02:16:37 +0100
From:   Stefan Agner <stefan@agner.ch>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     minchan@kernel.org, ngupta@vflare.org,
        sergey.senozhatsky.work@gmail.com, sjenning@linux.vnet.ibm.com,
        gregkh@linuxfoundation.org, arnd@arndb.de, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/zsmalloc: include sparsemem.h for MAX_PHYSMEM_BITS
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 <20201107165645.1b139b595b6b64feaca61bcb@linux-foundation.org>
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On 2020-11-08 01:56, Andrew Morton wrote:
> On Sat,  7 Nov 2020 16:22:06 +0100 Stefan Agner <stefan@agner.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?

Not sure if I got that right, but from what I understand if
MAX_PHYSMEM_BITS is not set in mm/zsmalloc.c it will set
MAX_PHYSMEM_BITS to BITS_PER_LONG. And this is 32-bit, too short when
LPAE is in use...

--
Stefan