Re: [PATCH] mm: define pte_add_end for consistency

[David Hildenbrand]

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.

-- 
Thanks,

David / dhildenb