Commit c04bf723 authored by Vincent Donnefort's avatar Vincent Donnefort Committed by Oliver Upton
Browse files

KVM: arm64: Do not transfer page refcount for THP adjustment 



GUP affects a refcount common to all pages forming the THP. There is
therefore no need to move the refcount from a tail to the head page.
Under the hood it decrements and increments the same counter.

Signed-off-by: default avatarVincent Donnefort <vdonnefort@google.com>
Reviewed-by: default avatarGavin Shan <gshan@redhat.com>
Reviewed-by: default avatarMarc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230928173205.2826598-2-vdonnefort@google.com


Signed-off-by: default avatarOliver Upton <oliver.upton@linux.dev>
parent 909b583f

arch/arm64/kvm/mmu.c

+0 −20
Original line number Diff line number Diff line
@@ -1295,28 +1295,8 @@ transparent_hugepage_adjust(struct kvm *kvm, struct kvm_memory_slot *memslot, 
		if (sz < PMD_SIZE)

			return PAGE_SIZE;



		/*

		 * The address we faulted on is backed by a transparent huge

		 * page.  However, because we map the compound huge page and

		 * not the individual tail page, we need to transfer the

		 * refcount to the head page.  We have to be careful that the

		 * THP doesn't start to split while we are adjusting the

		 * refcounts.

		 *

		 * We are sure this doesn't happen, because mmu_invalidate_retry

		 * was successful and we are holding the mmu_lock, so if this

		 * THP is trying to split, it will be blocked in the mmu

		 * notifier before touching any of the pages, specifically

		 * before being able to call __split_huge_page_refcount().

		 *

		 * We can therefore safely transfer the refcount from PG_tail

		 * to PG_head and switch the pfn from a tail page to the head

		 * page accordingly.

		 */

		*ipap &= PMD_MASK;

		kvm_release_pfn_clean(pfn);

		pfn &= ~(PTRS_PER_PMD - 1);

		get_page(pfn_to_page(pfn));

		*pfnp = pfn;



		return PMD_SIZE;