Re: [PATCH] mm: define pte_add_end for consistency

[David Hildenbrand]

On 03.07.20 03:34, Wei Yang wrote:
> On Thu, Jul 02, 2020 at 06:28:19PM +0200, David Hildenbrand wrote:
>> On 01.07.20 13:54, Wei Yang wrote:
>>> On Wed, Jul 01, 2020 at 10:29:08AM +0200, David Hildenbrand wrote:
>>>> On 01.07.20 04:11, Wei Yang wrote:
>>>>> On Tue, Jun 30, 2020 at 02:44:00PM +0200, David Hildenbrand wrote:
>>>>>> On 30.06.20 05:18, Wei Yang wrote:
>>>>>>> When walking page tables, we define several helpers to get the address of
>>>>>>> the next boundary. But we don't have one for pte level.
>>>>>>>
>>>>>>> Let's define it and consolidate the code in several places.
>>>>>>>
>>>>>>> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
>>>>>>> ---
>>>>>>>  arch/x86/mm/init_64.c   | 6 ++----
>>>>>>>  include/linux/pgtable.h | 7 +++++++
>>>>>>>  mm/kasan/init.c         | 4 +---
>>>>>>>  3 files changed, 10 insertions(+), 7 deletions(-)
>>>>>>>
>>>>>>> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
>>>>>>> index dbae185511cd..f902fbd17f27 100644
>>>>>>> --- a/arch/x86/mm/init_64.c
>>>>>>> +++ b/arch/x86/mm/init_64.c
>>>>>>> @@ -973,9 +973,7 @@ remove_pte_table(pte_t *pte_start, unsigned long addr, unsigned long end,
>>>>>>>  
>>>>>>>  	pte = pte_start + pte_index(addr);
>>>>>>>  	for (; addr < end; addr = next, pte++) {
>>>>>>> -		next = (addr + PAGE_SIZE) & PAGE_MASK;
>>>>>>> -		if (next > end)
>>>>>>> -			next = end;
>>>>>>> +		next = pte_addr_end(addr, end);
>>>>>>>  
>>>>>>>  		if (!pte_present(*pte))
>>>>>>>  			continue;
>>>>>>> @@ -1558,7 +1556,7 @@ void register_page_bootmem_memmap(unsigned long section_nr,
>>>>>>>  		get_page_bootmem(section_nr, pud_page(*pud), MIX_SECTION_INFO);
>>>>>>>  
>>>>>>>  		if (!boot_cpu_has(X86_FEATURE_PSE)) {
>>>>>>> -			next = (addr + PAGE_SIZE) & PAGE_MASK;
>>>>>>> +			next = pte_addr_end(addr, end);
>>>>>>>  			pmd = pmd_offset(pud, addr);
>>>>>>>  			if (pmd_none(*pmd))
>>>>>>>  				continue;
>>>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>>>>> index 32b6c52d41b9..0de09c6c89d2 100644
>>>>>>> --- a/include/linux/pgtable.h
>>>>>>> +++ b/include/linux/pgtable.h
>>>>>>> @@ -706,6 +706,13 @@ static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
>>>>>>>  })
>>>>>>>  #endif
>>>>>>>  
>>>>>>> +#ifndef pte_addr_end
>>>>>>> +#define pte_addr_end(addr, end)						\
>>>>>>> +({	unsigned long __boundary = ((addr) + PAGE_SIZE) & PAGE_MASK;	\
>>>>>>> +	(__boundary - 1 < (end) - 1) ? __boundary : (end);		\
>>>>>>> +})
>>>>>>> +#endif
>>>>>>> +
>>>>>>>  /*
>>>>>>>   * When walking page tables, we usually want to skip any p?d_none entries;
>>>>>>>   * and any p?d_bad entries - reporting the error before resetting to none.
>>>>>>> diff --git a/mm/kasan/init.c b/mm/kasan/init.c
>>>>>>> index fe6be0be1f76..89f748601f74 100644
>>>>>>> --- a/mm/kasan/init.c
>>>>>>> +++ b/mm/kasan/init.c
>>>>>>> @@ -349,9 +349,7 @@ static void kasan_remove_pte_table(pte_t *pte, unsigned long addr,
>>>>>>>  	unsigned long next;
>>>>>>>  
>>>>>>>  	for (; addr < end; addr = next, pte++) {
>>>>>>> -		next = (addr + PAGE_SIZE) & PAGE_MASK;
>>>>>>> -		if (next > end)
>>>>>>> -			next = end;
>>>>>>> +		next = pte_addr_end(addr, end);
>>>>>>>  
>>>>>>>  		if (!pte_present(*pte))
>>>>>>>  			continue;
>>>>>>>
>>>>>>
>>>>>> I'm not really a friend of this I have to say. We're simply iterating
>>>>>> over single pages, not much magic ....
>>>>>
>>>>> Hmm... yes, we are iterating on Page boundary, while we many have the case
>>>>> when addr or end is not PAGE_ALIGN.
>>>>
>>>> I really do wonder if not having page aligned addresses actually happens
>>>> in real life. Page tables operate on page granularity, and
>>>> adding/removing unaligned parts feels wrong ... and that's also why I
>>>> dislike such a helper.
>>>>
>>>> 1. kasan_add_zero_shadow()/kasan_remove_zero_shadow(). If I understand
>>>> the logic (WARN_ON()) correctly, we bail out in case we would ever end
>>>> up in such a scenario, where we would want to add/remove things not
>>>> aligned to PAGE_SIZE.
>>>>
>>>> 2. remove_pagetable()...->remove_pte_table()
>>>>
>>>> vmemmap_free() should never try to de-populate sub-pages. Even with
>>>> sub-section hot-add/remove (2MB / 512 pages), with valid struct page
>>>> sizes (56, 64, 72, 80), we always end up with full pages.
>>>>
>>>> kernel_physical_mapping_remove() is only called via
>>>> arch_remove_memory(). That will never remove unaligned parts.
>>>>
>>>
>>> I don't have a very clear mind now, while when you look into
>>> remove_pte_table(), it has two cases based on alignment of addr and next.
>>>
>>> If we always remove a page, the second case won't happen?
>>
>> So, the code talks about that the second case can only happen for
>> vmemmap, never for direct mappings.
>>
>> I don't see a way how this could ever happen with current page sizes,
>> even with sub-section hotadd (2MB). Maybe that is a legacy leftover or
>> was never relevant? Or I am missing something important, where we could
>> have sub-4k-page vmemmap data.
>>
> 
> I took a calculation on the sub-section page struct size, it is page size (4K)
> aligned. This means you are right, which we won't depopulate a sub-page.
> 
> And yes, I am not sure all those variants would fit this case. So I would like
> to leave as it now. How about your opinion?

I'd say we clean this up and protect it by WARN_ON_ONCE(). Then, it
won't need another round of investigation to find out that handling
sub-pages is irrelevant.

If you don't want to tackle this, I can have a look. Just let me know.

-- 
Thanks,

David / dhildenb