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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

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.

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