Re: [PATCH] mm: define pte_add_end for consistency

[Wei Yang]

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?

>3. register_page_bootmem_memmap()
>
>It operates on full pages only.
>
>
>This needs in-depth analysis, but my gut feeling is that this alignment
>is unnecessary.
>
>> 
>>>
>>> What would definitely make sense is replacing (addr + PAGE_SIZE) &
>>> PAGE_MASK; by PAGE_ALIGN() ...
>>>
>> 
>> No, PAGE_ALIGN() is expanded to be 
>> 
>> 	(addr + PAGE_SIZE - 1) & PAGE_MASK;
>> 
>> If we change the code to PAGE_ALIGN(), we would end up with infinite loop.
>
>Very right, it would have to be PAGE_ALIGN(addr + 1).
>
>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
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