Re: [PATCH v2 4/5] mm: FLEXIBLE_THP for improved performance

[Ryan Roberts <ryan.roberts@arm.com>]

On 05/07/2023 00:57, Matthew Wilcox wrote:
> On Tue, Jul 04, 2023 at 03:20:35PM +0100, Ryan Roberts wrote:
>> On 04/07/2023 04:45, Yin, Fengwei wrote:
>>>
>>> On 7/3/2023 9:53 PM, Ryan Roberts wrote:
>>>> Introduce FLEXIBLE_THP feature, which allows anonymous memory to be
>>> THP is for huge page which is 2M size. We are not huge page here. But
>>> I don't have good name either.
>>
>> Is that really true? On arm64 with 16K pages, huge pages are 32M and with 64K
>> base page, they are 512M. So huge pages already have a variable size. And they
>> sometimes get PTE-mapped. So can't we just think of this as an extension of the
>> THP feature?
> 
> The confusing thing is that we have counters for the number of THP
> allocated (and number of THP mapped), and for those we always use
> PMD-size folios.

OK fair point. I really don't have a strong opinion on the name - I changed it
from LARGE_ANON_FOLIO because Yu was suggesting it should be tied to THP. So I'm
happy to change it back to LARGE_ANON_FOLIO (or something else) if that's the
concensus. But I expect I'll end up in a game of ping-pong. So I'm going to keep
this name for now and focus on converging the actual implementation to something
that is agreeable. Once we are there, we can argue about the name.

> 
> If we must have a config option, then this is ANON_LARGE_FOLIOS.
> 
> But why do we need a config option?  We don't have one for the
> page cache, and we're better off for it.  Yes, it depends on
> CONFIG_TRANSPARENT_HUGEPAGE today, but that's more of an accidental
> heritage, and it'd be great to do away with that dependency eventually.
> 
> Hardware support isn't needed.  Large folios benefit us from a software
> point of view.  if we need a chicken bit, we can edit the source code
> to not create anon folios larger than order 0.

From my PoV it's about managing risk; there are currently parts of the mm that
will interact poorly with large pte-mapped folios (madvise, compaction, ...). We
want to incrementally fix that stuff, but until it's all fixed, we can't deploy
this as always-on. Further down the line when things are more complete and there
is more test coverage, we could remove the Kconfig or default it to enabled.