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

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Message-ID: <4a76fbda-aa9d-867b-e2eb-a1951780aeec@arm.com>
Date:   Mon, 19 Apr 2021 15:00:14 +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 19/04/2021 14:14, 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:
>>>>
>>>> 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 ;)
>>
> 
> Looking at how the generic ptdump code handles KASAN, I'm a bit sceptic.
> 
> IIUC, it is checking that kasan_early_shadow_pte is in the same page as 
> the pgtable referred by the PMD entry. But what happens if that PMD 
> entry is referring another pgtable which is inside the same page as 
> kasan_early_shadow_pte ?
> 
> Shouldn't the test be
> 
>      if (pmd_page_vaddr(val) == lm_alias(kasan_early_shadow_pte))
>          return note_kasan_page_table(walk, addr);

Now I come to look at this code again, I think you're right. On arm64 
this doesn't cause a problem - page tables are page sized and page 
aligned, so there couldn't be any non-KASAN pgtables sharing the page. 
Obviously that's not necessarily true of other architectures.

Feel free to add a patch to your series ;)

Steve