linux-kernel - Re: [PATCH v2] ARM: mm: support big-endian page tables

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Date:	Wed, 16 Apr 2014 13:28:44 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Jianguo Wu <wujianguo@...wei.com>
CC:	Will Deacon <Will.Deacon@....com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	Wang Nan <wangnan0@...wei.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Li Zefan <lizefan@...wei.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] ARM: mm: support big-endian page tables

On 16/04/14 03:45, Jianguo Wu wrote:
> On 2014/4/14 19:14, Marc Zyngier wrote:
> 
>> On 14/04/14 11:43, Will Deacon wrote:
>>> (catching up on old email)
>>>
>>> On Tue, Mar 18, 2014 at 07:35:59AM +0000, Jianguo Wu wrote:
>>>> Cloud you please take a look at this?
>>>
>>> [...]
>>>
>>>> On 2014/2/17 15:05, Jianguo Wu wrote:
>>>>> When enable LPAE and big-endian in a hisilicon board, while specify
>>>>> mem=384M mem=512M@...0M, will get bad page state:
>>>>>
>>>>> Freeing unused kernel memory: 180K (c0466000 - c0493000)
>>>>> BUG: Bad page state in process init  pfn:fa442
>>>>> page:c7749840 count:0 mapcount:-1 mapping:  (null) index:0x0
>>>>> page flags: 0x40000400(reserved)
>>>>> Modules linked in:
>>>>> CPU: 0 PID: 1 Comm: init Not tainted 3.10.27+ #66
>>>>> [<c000f5f0>] (unwind_backtrace+0x0/0x11c) from [<c000cbc4>] (show_stack+0x10/0x14)
>>>>> [<c000cbc4>] (show_stack+0x10/0x14) from [<c009e448>] (bad_page+0xd4/0x104)
>>>>> [<c009e448>] (bad_page+0xd4/0x104) from [<c009e520>] (free_pages_prepare+0xa8/0x14c)
>>>>> [<c009e520>] (free_pages_prepare+0xa8/0x14c) from [<c009f8ec>] (free_hot_cold_page+0x18/0xf0)
>>>>> [<c009f8ec>] (free_hot_cold_page+0x18/0xf0) from [<c00b5444>] (handle_pte_fault+0xcf4/0xdc8)
>>>>> [<c00b5444>] (handle_pte_fault+0xcf4/0xdc8) from [<c00b6458>] (handle_mm_fault+0xf4/0x120)
>>>>> [<c00b6458>] (handle_mm_fault+0xf4/0x120) from [<c0013754>] (do_page_fault+0xfc/0x354)
>>>>> [<c0013754>] (do_page_fault+0xfc/0x354) from [<c0008400>] (do_DataAbort+0x2c/0x90)
>>>>> [<c0008400>] (do_DataAbort+0x2c/0x90) from [<c0008fb4>] (__dabt_usr+0x34/0x40)
>>>
>>> [...]
>>>
>>>>> The bug is happened in cpu_v7_set_pte_ext(ptep, pte):
>>>>> when pte is 64-bit, for little-endian, will store low 32-bit in r2,
>>>>> high 32-bit in r3; for big-endian, will store low 32-bit in r3,
>>>>> high 32-bit in r2, this will cause wrong pfn stored in pte,
>>>>> so we should exchange r2 and r3 for big-endian.
>>>
> 
> Hi Marc,
> How about this:
> 
> The bug is happened in cpu_v7_set_pte_ext(ptep, pte):
> - It tests the L_PTE_NONE in one word on the other, and possibly clear L_PTE_VALID
>   tst	r3, #1 << (57 - 32)		@ L_PTE_NONE
>   bicne	r2, #L_PTE_VALID
> - Same for L_PTE_DIRTY, respectively setting L_PTE_RDONLY
> 
> As for LPAE, the pte is 64-bits, and the value of r2/r3 is depending on the endianness,
> for little-endian, will store low 32-bit in r2, high 32-bit in r3,
> for big-endian, will store low 32-bit in r3, high 32-bit in r2, 
> this will cause wrong bit is cleared or set, and get wrong pfn.
> So we should exchange r2 and r3 for big-endian.

May I suggest the following instead:

"An LPAE PTE is a 64bit quantity, passed to cpu_v7_set_pte_ext in the
 r2 and r3 registers.
 On an LE kernel, r2 contains the LSB of the PTE, and r3 the MSB.
 On a BE kernel, the assignment is reversed.

 Unfortunately, the current code always assumes the LE case,
 leading to corruption of the PTE when clearing/setting bits.

 This patch fixes this issue much like it has been done already in the
 cpu_v7_switch_mm case."

Cheers,

	M.
-- 
Jazz is not dead. It just smells funny...
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