Re: [PATCH v1 0/5] mm, kpageflags: support folio and fix output for compound pages

[David Hildenbrand <david@redhat.com>]

>>>> It does sound inconsistent. What exactly do you want to tell user space with
>>>> the new flag?
>>>
>>> The current most problematic behavior is to report folio as thp (order-2
>>> pagecache page is definitely a folio but not a thp), and this is what the
>>> new flag is intended to tell.
>>
>> We are currently considering calling these sub-PMD sized THPs "small-sized
>> THP". [1] Arguably, we're starting with the anon part where we won't get
>> around exposing them to the user in sysfs.
>>
>> So I wouldn't immediately say that these things are not THPs. They are not
>> PMD-sized THP. A slab/hugetlb is certainly not a thp but a folio. Whereby
>> slabs can also be order-0 folios, but hugetlb can't.
> 
> I think this is a mistake.  Users expect THPs to be PMD sized.  We already
> have the term "large folio" in use for file-backed memory; why do we
> need to invent a new term for anon large folios?

I changed my opinion two times, but I stabilized at "these are just huge 
pages of different size" when it comes to user-visible features.

Handling/calling them folios internally -- especially to abstract the 
page vs. compound page and how we manage/handle the metadata -- is a 
reasonable thing to do, because that's what we decided to pass around.


For future reference, here is a writeup about my findings and the reason 
for my opinion:


(1) OS-independent concept

Ignoring how the OS manages metadata (e.g., "struct page", "struct 
folio", compound head/tail, memdesc, ...), the common term to describe a 
"the smallest fixed-length contiguous block of physical memory into 
which memory pages are mapped by the operating system.["[1] is a page 
frame -- people usually simplify by dropping the "frame" part, so do I.

Larger pages (which we call "huge pages", FreeBSD "superpages", Windows 
"large pages") can come in different sizes and were traditionally based 
on architecture support, whereby architectures can support multiple ones 
[1]; I think what we see is that the OS might use intermediate sizes to 
manage memory more efficiently, abstracting/evolving that concept from 
the actual hardware page table mapping granularity.

But the foundation is that we are dealing with "blocks of physical 
memory" in a unit that is larger than the smallest page sizes. Larger pages.

[the comment about SGI IRIX on [1] is an interesting read; so are 
"scattered superpages"[3]]

Users learned the difference between a "page" and a "huge page". I'm 
confident that they can learn the difference between a "traditional huge 
page" and a "small-sized huge page", just like they did with hugetlb 
(below).

We just have to be careful with memory statistics and to default to the 
traditional huge pages for now. Slowly, the term "THP" will become more 
generic. Apart from that, I fail to see the big source of confusion.

Note: FreeBSD currently similarly calls these things on arm64 
"medium-sized superpages", and did not invent new terms for that so far 
[2].


(2) hugetlb

Traditional huge pages started out to be PMD-sized. Before 2008, we only 
supported a single huge page size. Ever since, we added support for 
sizes larger (gigantic) and smaller than that (cont-pte / cont-pmd).

So (a) users did not panic because we also supported huge pages that 
were not PMD-sized; (b) we managed to integrate it into the existing 
environment, defaulting to the old PMD-sized huge pages towards the user 
but still providing configuration knobs and (c) it is natural today to 
have multiple huge page sizes supported in hugetlb.

Nowadays, when somebody says that they are using hugetlb huge pages, the 
first question frequently is "which huge page size?". The same will 
happen with transparent huge pages I believe.


(3) THP preparation for multiple sizes

With
	/sys/kernel/mm/transparent_hugepage/hpage_pmd_size
added in 2016, we already provided a way for users to query the PMD size 
for THP, implying that there might be multiple sizes in the future.

Therefore, in commit 49920d28781d, Hugh already envisioned " some 
transparent support for pud and pgd pages" and ended up calling it 
"_pmd_size". Turns out, we want smaller THPs first, not larger ones.


(4) Metadata management

How the OS manages metadata for its memory -- and how it calls the 
involved datastructures -- is IMHO an implementation detail (an 
important one regarding performance, robustness and metadata overhead as 
we learned, though ;) ).

We were able to introduce folios without user-visible changes. We should 
be able to implement memdesc (or memory type hierarchies) without 
user-visible changes -- except for some interfaces that provide access 
to bare "struct page" information (classifies as debugging interfaces IMHO).


Last but not least, we ended up consistently calling these "larger than 
a page" things that we map into user space "(transparent) huge page" 
towards the user in toggles, stats and documentation. Fortunately we 
didn't use the term "compound page" back then; it would have been a mistake.


Regarding the pagecache, we managed to not expose any toggles towards 
the user, because memory waste can be better controlled. So the term 
"folio" does not pop up as a toggle in /sys and /proc.

	t14s: ~  $ find /sys -name "*folio*" 2> /dev/null
	t14s: ~  $ find /proc -name "*folio*" 2> /dev/null

Once we want to remove the (sub)page mapcount, we'll likely have to 
remove _nr_pages_mapped. To make some workloads that are sensitive to 
memory consumption [4] play along when not accounting only the actually 
mapped parts, we might have to introduce other ways to control that, 
when "/sys/kernel/debug/fault_around_bytes" no longer does the trick. 
I'm hoping we can still find ways to avoid exposing any toggles for 
that; we'll see.


[1] https://en.wikipedia.org/wiki/Page_(computer_memory)
[2] https://www.freebsd.org/status/report-2022-04-2022-06/superpages/
[3] https://ieeexplore.ieee.org/document/6657040/similar#similar
[4] https://www.suse.com/support/kb/doc/?id=000019017


> 
>> Looking at other interfaces, we do expose:
>>
>> include/uapi/linux/kernel-page-flags.h:#define KPF_COMPOUND_HEAD        15
>> include/uapi/linux/kernel-page-flags.h:#define KPF_COMPOUND_TAIL        16
>>
>> So maybe we should just continue talking about compound pages or do we have
>> to use both terms here in this interface?
> 
> I don;t know how easy it's going to be to distinguish between a head
> and tail page in the Glorious Future once pages and folios are separated.

Probably a page-based interface would be the wrong interface for that; 
fortunately, this interface has a "debugging" smell to it, so we might 
be able to replace it.

-- 
Cheers,

David / dhildenb