Re: [PATCH v9 00/21] Generic page walk and ptdump

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 07/25/2019 03:00 PM, Will Deacon wrote:
> On Thu, Jul 25, 2019 at 02:39:22PM +0530, Anshuman Khandual wrote:
>> On 07/24/2019 07:05 PM, Steven Price wrote:
>>> There isn't any problem as such with using p?d_large macros. However the
>>> name "large" has caused confusion in the past. In particular there are
>>> two types of "large" page:
>>>
>>> 1. leaf entries at high levels than normal ('sections' on Arm, for 4K
>>> pages this gives you 2MB and 1GB pages).
>>>
>>> 2. sets of contiguous entries that can share a TLB entry (the
>>> 'Contiguous bit' on Arm - which for 4K pages gives you 16 entries = 64
>>> KB 'pages').
>>
>> This is arm64 specific and AFAIK there are no other architectures where there
>> will be any confusion wrt p?d_large() not meaning a single entry.
>>
>> As you have noted before if we are printing individual entries with PTE_CONT
>> then they need not be identified as p??d_large(). In which case p?d_large()
>> can just safely point to p?d_sect() identifying regular huge leaf entries.
> 
> Steven's stuck in the middle of things here, but I do object to p?d_large()
> because I find it bonkers to have p?d_large() and p?d_huge() mean completely
> different things when they are synonyms in the English language.

Agreed that both p?d_large() and p?d_huge() should not exist at the same time
when they imply the same thing. I believe all these name proliferation happened
because THP, HugeTLB and kernel large mappings like linear, vmemmap, ioremap etc
which the platform code had to deal with in various forms.

> 
> Yes, p?d_leaf() matches the terminology used by the Arm architecture, but
> given that most page table structures are arranged as a 'tree', then it's
> not completely unreasonable, in my opinion. If you have a more descriptive
> name, we could use that instead. We could also paint it blue.

The alternate name chosen p?d_leaf() is absolutely fine and it describes the
entry as intended. There is no disagreement over that. My original concern
was introduction of yet another page table helper.

> 
>>> In many cases both give the same effect (reduce pressure on TLBs and
>>> requires contiguous and aligned physical addresses). But for this case
>>> we only care about the 'leaf' case (because the contiguous bit makes no
>>> difference to walking the page tables).
>>
>> Right and we can just safely identify section entries with it. What will be
>> the problem with that ? Again this is only arm64 specific.
>>
>>>
>>> As far as I'm aware p?d_large() currently implements the first and
>>> p?d_(trans_)huge() implements either 1 or 2 depending on the architecture.
>>
>> AFAIK option 2 exists only on arm6 platform. IIUC generic MM requires two
>> different huge page dentition from platform. HugeTLB identifies large entries
>> at PGD|PUD|PMD after converting it's content into PTE first. So there is no
>> need for direct large page definitions for other levels.
>>
>> 1. THP		- pmd_trans_huge()
>> 2. HugeTLB	- pte_huge()	   CONFIG_ARCH_WANT_GENERAL_HUGETLB is set
>>
>> A simple check for p?d_large() on mm/ and include/linux shows that there are
>> no existing usage for these in generic MM. Hence it is available.
> 
> Alternatively, it means we have a good opportunity to give it a better name
> before it spreads into the core code.

Fair enough, that is another way. So you expect existing platform definitions
for p?d_large()/p?d_huge() getting cleaned up and to start using new p?d_leaf()
instead ?

> 
>> IMHO the new addition of p?d_leaf() can be avoided and p?d_large() should be
>> cleaned up (if required) in platforms and used in generic functions.
> 
> Again, I disagree and think p?d_large() should be confined to arch code
> if it sticks around at all.

All of those instances should migrate to using p?d_leaf() eventually else
there will be three different helpers which probably mean the same thing.