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

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Date:   Sat, 7 Nov 2020 20:54:13 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Stefan Agner <stefan@...er.ch>,
        Andrew Morton <akpm@...ux-foundation.org>
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 11/7/2020 5:16 PM, Stefan Agner wrote:
> On 2020-11-08 01:56, Andrew Morton wrote:
>> 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?
> 
> 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...

You should probably include this in your commit message since that is
describing the bug and then your fix makes complete sense.
-- 
Florian