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

David Hildenbrand
2023-10-16T10:14:27Z

>>>> 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