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

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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@google.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230928173205.2826598-2-vdonnefort@google.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
This commit is contained in:
Vincent Donnefort 2023-09-28 18:32:03 +01:00 committed by Oliver Upton
parent 909b583f81
commit c04bf723cc
1 changed files with 0 additions and 20 deletions
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20
arch/arm64/kvm/mmu.c
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@ -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;