Re: [PATCH mm-unstable v18 10/14] mm/khugepaged: introduce collapse_allowable_orders helper function

On 6/1/26 16:35, Lorenzo Stoakes wrote:

On Sun, May 31, 2026 at 10:18:16PM +0200, David Hildenbrand (Arm) wrote:

quoted

On 5/22/26 17:00, Nico Pache wrote:

quoted

Add collapse_allowable_orders() to generalize THP order eligibility. The
function determines which THP orders are permitted based on collapse
context (khugepaged vs madv_collapse).

This consolidates collapse configuration logic and provides a clean
interface for future mTHP collapse support where the orders may be
different.

It would have been good to describe here that, for now, it only ever returns
PMDs, and that it will be extended next.

Logically, this patch belongs to #12, not #11 ... so seeing it before #11 was a bit

... and there, it is clear that we don't even want to know the orders?

So can we just call this function

"collapse_possible" and make it return a boolean?

FWIW, I realized later that #11 has

enabled_orders = collapse_allowable_orders(vma, vma->vm_flags, tva_flags);

and forgot to delete that comment.

So we must have a variant (for #11) that returns the enabled orders.

Yeah agreed.

But I also don't love the naming, we now have thp_vma_allowable_orders(),
__thp_vma_allowable_orders(), and then collapse_allowable_orders() and we also
have 3 different ways of collapsing, one of which we call MADV_... COLLAPSE,
and the other khugepaged + fault-in too for laughs.

It's like a big circle of confusion.

And of course we call THP collapse 'collapse' in general, so :)

Anyway I'm fine with collapse_possible() so we can move on and then maybe
cleanup later.

We could simply have

collapse_possible_orders()

and

collapse_possible()

the latter being a simple wrapper around collapse_possible_orders().

Also - if I look at khugepaged.c I see thp_vma_allowable_orders() still used in
hugepage_vma_revalidate().

Wouldn't it be better then to do the abstraction once mTHP order checking is
properly introduced and change this also and have _every_ order check be
consistent in khugepaged.c?

That'd also be nice, if easily possible.

-- 
Cheers,

David