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

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Date:   Fri, 16 Apr 2021 12:38:18 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Steven Price <steven.price@....com>,
        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()



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/


Christophe