Re: [PATCH v2 1/2] powerpc: Fix virt_addr_valid() check

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

Le 19/01/2022 à 02:15, Kefeng Wang a écrit :
> 
> On 2022/1/11 14:04, Christophe Leroy wrote:
>>
>> Le 11/01/2022 à 05:37, Nicholas Piggin a écrit :
>>> Excerpts from Kefeng Wang's message of January 8, 2022 9:58 pm:
>>>> Hi PPC maintainers， ping..
>>> Hmm. I might have confused myself about this. I'm going back and
>>> trying to work out what I was thinking when I suggested it. This
>>> works on 64e because vmalloc space is below the kernel linear map,
>>> right?
>>>
>>> On 64s it is the other way around and it is still possible to enable
>>> flatmem on 64s. Altough we might just not hit the problem there because
>>> __pa() will not mask away the vmalloc offset for 64s so it will still
>>> return something that's outside the pfn_valid range for flatmem. That's
>>> very subtle though.
>> That's the way it works on PPC32 at least, so for me it's not chocking
>> to have it work the same way on PPC64s.
>>
>> The main issue here is the way __pa() works. On PPC32 __pa = va -
>> PAGE_OFFSET, so it works correctly for any address.
>> On PPC64, __pa() works by masking out the 2 top bits instead of
>> substracting PAGE_OFFSET, so the test must add a verification that we
>> really have the 2 top bits set at first. This is what (addr >=
>> PAGE_OFFSET) does. Once this first test is done, we can perfectly rely
>> on pfn_valid() just like PPC32, I see absolutely no point in an
>> additionnal test checking the addr is below KERN_VIRT_START.
> 
> 
> Hi Christophe and Nicholas， for ppc32， I think we need check the upper 
> limit,

Why ? Have you experimented any problem at all on PPC32 with the way it 
is done at the moment ?

I don't think we have to change PPC32 at all unless we have a real 
reason to do it.

> 
> eg,  addr >= PAGE_OFFSET && addr < high_memory

Isn't it exactly what pfn_valid() already do today ?
Why change that at all ?

Christophe

> 
> arch/powerpc/mm/mem.c:  high_memory = (void *) __va(max_low_pfn * 
> PAGE_SIZE);
> 
> for ppc32 max_low_pfn is the upper low memory pfn,  and For ppc64, 
> high_memory is
> 
> the max memory pfn, it looks good too, correct me if I'm wrong, if the 
> above check
> 
> is ok, I will send a new v3,  thanks.
> 
> 
> 
> 
>>
>>
>>> The checks added to __pa actually don't prevent vmalloc memory from
>>> being passed to it either on 64s, only a more basic test.
>> That's correct. It is the role of pfn_valid() to check that.
>>
>> Christophe
>>
>>> I think 64s wants (addr >= PAGE_OFFSET && addr < KERN_VIRT_START) as
>>> the condition.  Could possibly add that check to __pa as well to
>>> catch vmalloc addresses.
>>>
>>> Thanks,
>>> Nick
>>>
>>>