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

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Message-ID: <10adad00-14de-61b6-ce2a-bdde23a34bcf@csgroup.eu>
Date:   Fri, 16 Apr 2021 17:15:23 +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, linux-s390@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-riscv@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v1 3/5] mm: ptdump: Provide page size to notepage()



Le 16/04/2021 à 17:04, Christophe Leroy a écrit :
> 
> 
> Le 16/04/2021 à 16:40, Christophe Leroy a écrit :
>>
>>
>> Le 16/04/2021 à 15:00, Steven Price a écrit :
>>> 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 :
>>>>>>> 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.
>>>
>>
>> I'm trying. I reverted the two commits b00ff6d8c and cabe8138.
>>
>> At the moment, I don't get exactly what I expect: For linear memory I get one line for each 8M 
>> page whereas before reverting the patches I got one 16M line and one 112M line.
>>
>> And for KASAN shadow area I get two lines for the 2x 8M pages shadowing linear mem then I get one 
>> 4M line for each PGDIR entry pointing to kasan_early_shadow_pte.
>>
>> 0xf8000000-0xf87fffff 0x07000000         8M   huge        rw       present
>> 0xf8800000-0xf8ffffff 0x07800000         8M   huge        rw       present
>> 0xf9000000-0xf93fffff 0x01430000         4M               r        present
> ...
>> 0xfec00000-0xfeffffff 0x01430000         4M               r        present
>>
>> Any idea ?
>>
> 
> 
> I think the different with other architectures is here:
> 
>      } else if (flag != st->current_flags || level != st->level ||
>             addr >= st->marker[1].start_address ||
>             pa != st->last_pa + PAGE_SIZE) {
> 
> 
> In addition to the checks everyone do, powerpc also checks "pa != st->last_pa + PAGE_SIZE".
> And it is definitely for that test that page_size argument add been added.

By replacing that test by (pa - st->start_pa != addr - st->start_address) it works again. So we 
definitely don't need the real page size.


> 
> I see that other architectures except RISCV don't dump the physical address. But even RISCV doesn't 
> include that check.
> 
> That physical address dump was added by commit aaa229529244 ("powerpc/mm: Add physical address to 
> Linux page table dump") [https://github.com/torvalds/linux/commit/aaa2295]
> 
> How do other architectures deal with the problem described by the commit log of that patch ?
> 
> Christophe