Re: [RESEND PATCH v7 00/10] Small-sized THP for anonymous memory

[David Hildenbrand <david@redhat.com>]

On 23.11.23 17:18, Matthew Wilcox wrote:
> On Thu, Nov 23, 2023 at 05:05:37PM +0100, David Hildenbrand wrote:
>> On 23.11.23 16:59, Matthew Wilcox wrote:
>>> On Wed, Nov 22, 2023 at 04:29:40PM +0000, Ryan Roberts wrote:
>>>> Note: I'm resending this at Andrew's suggestion due to having originally sent
>>>> it during LPC. I'm hoping its in a position where the feedback is minor enough
>>>> that I can rework in time for v6.8, but so far haven't had any.
>>>>
>>>> Hi All,
>>>>
>>>> This is v7 of a series to implement small-sized THP for anonymous memory
>>>> (previously called "large anonymous folios"). The objective of this is to
>>>
>>> I'm still against small-sized THP.  We've now got people asking whether
>>> the THP counters should be updated when dealing with large folios that
>>> are smaller than PMD sized.  It's sowing confusion, and we should go
>>> back to large anon folios as a name.
>>>
>>
>> I disagree.
>>
>> https://lore.kernel.org/all/65dbdf2a-9281-a3c3-b7e3-a79c5b60b357@redhat.com/
> 
> And yet:
> https://lore.kernel.org/linux-mm/20231106193315.GB3661273@cmpxchg.org/
> 
> "This is a small THP so we don't account it as a THP, we only account
> normal THPs as THPs" is a bizarre position to take.
>
> Not to mention that saying a foo is a small huge baz is just bizarre.
> Am I a small giant?  Or just a large human?

I like that analogy. Yet, "small giant" sounds "bigger" in some way IMHO ;)

I'll note that "small-sized THP" is just a temporary feature name, it 
won't be exposed as such to the user in sysfs etc. In a couple of years, 
it will be forgotten.

To me it makes sense: it's a hugepage (not a page) but smaller compared 
to what we previously had. But again, there won't be a "small_thp" 
toggle anywhere.

Long-term it's simply going to be a THP. Quoting from my writeup:

"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.".


Regarding the accounting: as I said a couple of times, "AnonHugePages" 
should have been called "AnonPmdMapped" or similar; that's what it 
really is: as soon as a THP is PTE-mapped, it's not accounted there. But 
we can't fix that I guess, unless we add some "world switch" for any 
workloads that would care about a different accounting.

So we're really only concerned about:
* AnonHugePages
* ShmemHugePages
* FileHugePages

The question is if we really want to continue extending/adjusting the 
old meminfo interfaces and talk about how to perform accounting there.

Because, as we learned, we might get a new file-based sysfs based 
interface, because Greg seems to be against exposing new values in the 
old single-file-based one.

In a new one, we have all freedom to expose what we actually want 
nowadays, and can just document that the old interface was designed with 
the assumption that there is only a single THP size.

... like hugetlb, where we also only expose the "default hugetlb size" 
parameters for legacy reasons:

HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB

-- 
Cheers,

David / dhildenb