Re: [PATCH 6/6] mm: rename ARCH_ENABLE_THP_MIGRATION to ARCH_SUPPORTS_PMD_SOFTLEAF

[Lorenzo Stoakes <ljs@kernel.org>]

On Wed, Jul 01, 2026 at 10:03:21PM +0200, David Hildenbrand (Arm) wrote:
> On 6/30/26 18:34, Usama Arif wrote:
> > CONFIG_ARCH_ENABLE_THP_MIGRATION started life gating just PMD-level
> > migration entries, but has grown to gate the entire PMD-level softleaf
> > machinery: migration entries, device-private entries, and soon swap
> > entries.
>
> device-private really relies on migration support.
>
> See do_huge_pmd_device_private() where we migrate back to CPU memory.
>
> The whole concept is pased on migration. So that's why it currently makes sense.
>
> So "has grown to" is not quite accurate. It's a different story with swap
> entries (that can exist even without migration support).
>
>
> > Rename CONFIG_ARCH_ENABLE_THP_MIGRATION to CONFIG_ARCH_SUPPORTS_PMD
> > _SOFTLEAF to make this clear.  This is a pure rename: the set of
> > selecting architectures (x86, arm64, s390, riscv, loongarch, and
> > powerpc on PPC_BOOK3S_64) and the gating semantics are unchanged.
>
> Hm, that makes me wonder what it would take to make this all just be implicitly
> supported by CONFIG_TRANSPARENT_HUGEPAGE.
>
> Looks like someone would have the courage to touch stuff like arc, mips and
> sparc64 (+arm and x86, but that's less of a concern :) ).
>
> So yeah, renaming the config option makes sense. The config space is the wild west.
>
> We have
>
> 	PGTABLE_HAS_HUGE_LEAVES
>
> and stuff like
>
> 	HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> 	HAVE_ARCH_SOFT_DIRTY
> 	ARCH_HAS_GIGANTIC_PAGE
> 	ARCH_HAS_PTE_SPECIAL
>
> We do have a limited number of "SUPPORTS" (which is longer than HAS), but I
> would either use
>
>
> 	ARCH_HAS_PMD_SOFTLEAVES
>
> or
>
> 	HAVE_ARCH_PMD_SOFTLEAVES

Yeah it'd be GREAT to simplify all this!

>
>
> Apart from that LGTM.

Can I just say I love this patch in general though? Softleaf all the things :))

>
> --
> Cheers,
>
> David

Cheers, Lorenzo