Re: [PATCH v1 2/2] mm: mTHP stats for pagecache folio allocations

[David Hildenbrand <david@redhat.com>]

>> Sorry, busy with other stuff.
>>
>> Indicating only what really exists sounds cleaner. But I wonder how we would
>> want to handle in general orders that are effectively non-existant?
> 
> I'm not following your distinction between orders that don't "really exist" and
> orders that are "effectively non-existant".

I'm questioning whether there should be a distinction at all. We should 
just hide what is either non-existant (not implemented) or non-functional.

> 
> I guess the real supported orders are:
> 
>    anon:
>      min order: 2
>      max order: PMD_ORDER
>    anon-shmem:
>      min order: 1
>      max order: MAX_PAGECACHE_ORDER
>    tmpfs-shmem:
>      min order: PMD_ORDER <= 11 ? PMD_ORDER : NONE
>      max order: PMD_ORDER <= 11 ? PMD_ORDER : NONE
>    file:
>      min order: 1
>      max order: MAX_PAGECACHE_ORDER

That's my understanding. But not sure about anon-shmem really supporting 
order-1, maybe we do.

> 
> But today, controls and stats are exposed for:
> 
>    anon:
>      min order: 2
>      max order: PMD_ORDER
>    anon-shmem:
>      min order: 2
>      max order: PMD_ORDER
>    tmpfs-shmem:
>      min order: PMD_ORDER
>      max order: PMD_ORDER
>    file:
>      min order: Nothing yet (this patch proposes 1)
>      max order: Nothing yet (this patch proposes MAX_PAGECACHE_ORDER)
> 
> So I think there is definitely a bug for shmem where the minimum order control
> should be order-1 but its currently order-2.

Maybe, did not play with that yet. Likely order-1 will work. (although 
probably of questionable use :) )

> 
> I also wonder about PUD-order for DAX? We don't currently have a stat/control.
> If we wanted to add it in future, if we take the "expose all stats/controls for
> all orders" approach, we would end up extending all the way to PUD-order and all
> the orders between PMD and PUD would be dummy for all memory types. That really
> starts to feel odd, so I still favour only populating what's really supported.

I would go further and say that calling the fsdax thing a THP is 
borderline wrong and we should not expose any new toggles for it that way.

It really behaves much more like hugetlb folios that can be PTE-mapped 
... we cannot split these things, and they are not allocated from the 
buddy. So I wouldn't worry about fsdax for now.

fsdax support for compound pages (now large folios) probably never 
should have been glued to any THP toggle.

> 
> I propose to fix shmem (extend down to 1, stop at MAX_PAGECACHE_ORDER) and
> continue with the approach of "indicating only what really exists" for v2.
> 
> Shout if you disagree.

Makes sense.

-- 
Cheers,

David / dhildenb