Re: [RESEND PATCH] mm: align larger anonymous mappings on THP boundaries

[David Hildenbrand]

On 07.05.24 13:26, Ryan Roberts wrote:
> On 07/05/2024 12:14, Ryan Roberts wrote:
> > On 07/05/2024 12:13, David Hildenbrand wrote:
> > > > https://github.com/intel/lmbench/blob/master/src/lat_mem_rd.c#L95
> > > >
> > > > > suggest. If you want to try something semi-randomly; it might be useful to rule
> > > > > out the arm64 contpte feature. I don't see how that would be interacting here if
> > > > > mTHP is disabled (is it?). But its new for 6.9 and arm64 only. Disable with
> > > > > ARM64_CONTPTE (needs EXPERT) at compile time.
> > > > I don't enabled mTHP, so it should be not related about ARM64_CONTPTE,
> > > > but will have a try.
> > >
> > > cont-pte can get active if we're just lucky when allocating pages in the right
> > > order, correct Ryan?
> >
> > No it shouldn't do; it requires the pages to be in the same folio.

Ah, my memory comes back. That's also important for folio_pte_batch() to 
currently work as expected I think. We could change that, though, and 
let cont-pte batch across folios.

> >
>
> That said, if we got lucky in allocating the "right" pages, then we will end up
> doing an extra function call and a bit of maths per every 16 PTEs in order to
> figure out that the span is not contained by a single folio, before backing out
> of an attempt to fold. That would probably be just about measurable.
>
> But the regression doesn't kick in until 96K, which is the step after 64K. I'd
> expect to see the regression on 64K too if that was the issue. The cacheline is
> 64K so I suspect it could be something related to the cache?

--
Cheers,

David / dhildenb