Date:   Fri, 3 Jul 2020 16:33:32 +0800
From:   Wei Yang <richard.weiyang@...ux.alibaba.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        akpm@...ux-foundation.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: define pte_add_end for consistency

On Fri, Jul 03, 2020 at 09:23:30AM +0200, David Hildenbrand wrote:
>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@...ux.alibaba.com>
>>>>>>>> ---
>>>>>>>>  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.
>

Actually, I don't get what you are trying to do. So go ahead, maybe I can
review your change.

>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
Help you, Help me

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