Re: [RFC PATCH v2 29/51] mm: guestmem_hugetlb: Wrap HugeTLB as an allocator for guest_memfd

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 03, 2025, Sean Christopherson wrote:
> On Wed, May 14, 2025, Ackerley Tng wrote:
> > guestmem_hugetlb is an allocator for guest_memfd. It wraps HugeTLB to
> > provide huge folios for guest_memfd.
> > 
> > This patch also introduces guestmem_allocator_operations as a set of
> > operations that allocators for guest_memfd can provide. In a later
> > patch, guest_memfd will use these operations to manage pages from an
> > allocator.
> > 
> > The allocator operations are memory-management specific and are placed
> > in mm/ so key mm-specific functions do not have to be exposed
> > unnecessarily.
> 
> This code doesn't have to be put in mm/, all of the #includes are to <linux/xxx.h>.
> Unless I'm missing something, what you actually want to avoid is _exporting_ mm/
> APIs, and for that all that is needed is ensure the code is built-in to the kernel
> binary, not to kvm.ko.
> 
> diff --git a/virt/kvm/Makefile.kvm b/virt/kvm/Makefile.kvm
> index d047d4cf58c9..c18c77e8a638 100644
> --- a/virt/kvm/Makefile.kvm
> +++ b/virt/kvm/Makefile.kvm
> @@ -13,3 +13,5 @@ kvm-$(CONFIG_HAVE_KVM_IRQ_ROUTING) += $(KVM)/irqchip.o
>  kvm-$(CONFIG_HAVE_KVM_DIRTY_RING) += $(KVM)/dirty_ring.o
>  kvm-$(CONFIG_HAVE_KVM_PFNCACHE) += $(KVM)/pfncache.o
>  kvm-$(CONFIG_KVM_GUEST_MEMFD) += $(KVM)/guest_memfd.o
> +
> +obj-$(subst m,y,$(CONFIG_KVM_GUEST_MEMFD)) += $(KVM)/guest_memfd_hugepages.o
> \ No newline at end of file
> 
> People may want the code to live in mm/ for maintenance and ownership reasons
> (or not, I haven't followed the discussions on hugepage support), but that's a
> very different justification than what's described in the changelog.
> 
> And if the _only_ user is guest_memfd, putting this in mm/ feels quite weird.
> And if we anticipate other users, the name guestmem_hugetlb is weird, because
> AFAICT there's nothing in here that is in any way guest specific, it's just a
> few APIs for allocating and accounting hugepages.
> 
> Personally, I don't see much point in trying to make this a "generic" library,
> in quotes because the whole guestmem_xxx namespace makes it anything but generic.
> I don't see anything in mm/guestmem_hugetlb.c that makes me go "ooh, that's nasty,
> I'm glad this is handled by a library".  But if we want to go straight to a
> library, it should be something that is really truly generic, i.e. not "guest"
> specific in any way.

Ah, the complexity and the mm-internal dependencies come along in the splitting
and merging patch.  Putting that code in mm/ makes perfect sense, but I'm still
not convinced that putting _all_ of this code in mm/ is the correct split.

As proposed, this is a weird combination of being an extension of guest_memfd, a
somewhat generic library, _and_ a subsystem (e.g. the global workqueue and stash).

_If_ we need a library, then IMO it should be a truly generic library.  Any pieces
that are guest_memfd specific belong in KVM.  And any subsystem-like things should
should probably be implemented as an extension to HugeTLB itself, which is already
it's own subsytem.  Emphasis on "if", because it's not clear to me that that a
library is warranted.

AFAICT, the novelty here is the splitting and re-merging of hugetlb folios, and
that seems like it should be explicitly an extension of the hugetlb subsystem.
E.g. that behavior needs to take hugetlb_lock, interact with global vmemmap state
like hugetlb_optimize_vmemmap_key, etc.  If that's implemented as something like
hugetlb_splittable.c or whatever, and wired up to be explicitly configured via
hugetlb_init(), then there may not be much left for a library.