linux-kernel - Re: [PATCH v1 3/5] mm: ptdump: Provide page size to notepage()

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Message-ID: <e39d500a-2154-3c5d-9393-8bf53a567fad@arm.com>
Date:   Fri, 16 Apr 2021 14:00:28 +0100
From:   Steven Price <steven.price@....com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        akpm@...ux-foundation.org
Cc:     linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, x86@...nel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 3/5] mm: ptdump: Provide page size to notepage()

On 16/04/2021 12:08, Christophe Leroy wrote:
> 
> 
> Le 16/04/2021 à 12:51, Steven Price a écrit :
>> On 16/04/2021 11:38, Christophe Leroy wrote:
>>>
>>>
>>> Le 16/04/2021 à 11:28, Steven Price a écrit :
>>>> On 15/04/2021 18:18, Christophe Leroy wrote:
>>>>> In order to support large pages on powerpc, notepage()
>>>>> needs to know the page size of the page.
>>>>>
>>>>> Add a page_size argument to notepage().
>>>>>
>>>>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>>>>> ---
>>>>>   arch/arm64/mm/ptdump.c         |  2 +-
>>>>>   arch/riscv/mm/ptdump.c         |  2 +-
>>>>>   arch/s390/mm/dump_pagetables.c |  3 ++-
>>>>>   arch/x86/mm/dump_pagetables.c  |  2 +-
>>>>>   include/linux/ptdump.h         |  2 +-
>>>>>   mm/ptdump.c                    | 16 ++++++++--------
>>>>>   6 files changed, 14 insertions(+), 13 deletions(-)
>>>>>
>>>> [...]
>>>>> diff --git a/mm/ptdump.c b/mm/ptdump.c
>>>>> index da751448d0e4..61cd16afb1c8 100644
>>>>> --- a/mm/ptdump.c
>>>>> +++ b/mm/ptdump.c
>>>>> @@ -17,7 +17,7 @@ static inline int note_kasan_page_table(struct 
>>>>> mm_walk *walk,
>>>>>   {
>>>>>       struct ptdump_state *st = walk->private;
>>>>> -    st->note_page(st, addr, 4, pte_val(kasan_early_shadow_pte[0]));
>>>>> +    st->note_page(st, addr, 4, pte_val(kasan_early_shadow_pte[0]), 
>>>>> PAGE_SIZE);
>>>>
>>>> I'm not completely sure what the page_size is going to be used for, 
>>>> but note that KASAN presents an interesting case here. We short-cut 
>>>> by detecting it's a KASAN region at a high level (PGD/P4D/PUD/PMD) 
>>>> and instead of walking the tree down just call note_page() *once* 
>>>> but with level==4 because we know KASAN sets up the page table like 
>>>> that.
>>>>
>>>> However the one call actually covers a much larger region - so while 
>>>> PAGE_SIZE matches the level it doesn't match the region covered. 
>>>> AFAICT this will lead to odd results if you enable KASAN on powerpc.
>>>
>>> Hum .... I successfully tested it with KASAN, I now realise that I 
>>> tested it with CONFIG_KASAN_VMALLOC selected. In this situation, 
>>> since https://github.com/torvalds/linux/commit/af3d0a686 we don't 
>>> have any common shadow page table anymore.
>>>
>>> I'll test again without CONFIG_KASAN_VMALLOC.
>>>
>>>>
>>>> To be honest I don't fully understand why powerpc requires the 
>>>> page_size - it appears to be using it purely to find "holes" in the 
>>>> calls to note_page(), but I haven't worked out why such holes would 
>>>> occur.
>>>
>>> I was indeed introduced for KASAN. We have a first commit 
>>> https://github.com/torvalds/linux/commit/cabe8138 which uses page 
>>> size to detect whether it is a KASAN like stuff.
>>>
>>> Then came https://github.com/torvalds/linux/commit/b00ff6d8c as a 
>>> fix. I can't remember what the problem was exactly, something around 
>>> the use of hugepages for kernel memory, came as part of the series 
>>> https://patchwork.ozlabs.org/project/linuxppc-dev/cover/cover.1589866984.git.christophe.leroy@csgroup.eu/ 
>>
>>
>>
>> Ah, that's useful context. So it looks like powerpc took a different 
>> route to reducing the KASAN output to x86.
>>
>> Given the generic ptdump code has handling for KASAN already it should 
>> be possible to drop that from the powerpc arch code, which I think 
>> means we don't actually need to provide page size to notepage(). 
>> Hopefully that means more code to delete ;)
>>
> 
> Yes ... and no.
> 
> It looks like the generic ptdump handles the case when several pgdir 
> entries points to the same kasan_early_shadow_pte. But it doesn't take 
> into account the powerpc case where we have regular page tables where 
> several (if not all) PTEs are pointing to the kasan_early_shadow_page .

I'm not sure I follow quite how powerpc is different here. But could you 
have a similar check for PTEs against kasan_early_shadow_pte as the 
other levels already have?

I'm just worried that page_size isn't well defined in this interface and 
it's going to cause problems in the future.

Steve