[PATCH v2 1/2] KVM: arm64: Do not transfer page refcount for THP adjustment

[Vincent Donnefort]

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: Vincent Donnefort <vdonnefort at google.com>

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 587a104f66c3..de5e5148ef5d 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -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;
-- 
2.42.0.515.g380fc7ccd1-goog