linux-kernel - [RFC PATCH V2 3/4] powerpc/mm/iommu: Allow large IOMMU page size only for hugetlb backing

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Date:   Thu,  6 Sep 2018 11:13:41 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     akpm@...ux-foundation.org, Alexey Kardashevskiy <aik@...abs.ru>,
        mpe@...erman.id.au
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Subject: [RFC PATCH V2 3/4] powerpc/mm/iommu: Allow large IOMMU page size only for hugetlb backing

THP pages can get split during different code paths. An incremented reference
count do imply we will not split the compound page. But the pmd entry can be
converted to level 4 pte entries. Keep the code simpler by allowing large
IOMMU page size only if the guest ram is backed by hugetlb pages.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
---
 arch/powerpc/mm/mmu_context_iommu.c | 16 ++--------------
 1 file changed, 2 insertions(+), 14 deletions(-)

diff --git a/arch/powerpc/mm/mmu_context_iommu.c b/arch/powerpc/mm/mmu_context_iommu.c
index c9ee9e23845f..f472965f7638 100644
--- a/arch/powerpc/mm/mmu_context_iommu.c
+++ b/arch/powerpc/mm/mmu_context_iommu.c
@@ -212,21 +212,9 @@ long mm_iommu_get(struct mm_struct *mm, unsigned long ua, unsigned long entries,
 		}
 populate:
 		pageshift = PAGE_SHIFT;
-		if (mem->pageshift > PAGE_SHIFT && PageCompound(page)) {
-			pte_t *pte;
+		if (mem->pageshift > PAGE_SHIFT && PageHuge(page)) {
 			struct page *head = compound_head(page);
-			unsigned int compshift = compound_order(head);
-			unsigned int pteshift;
-
-			local_irq_save(flags); /* disables as well */
-			pte = find_linux_pte(mm->pgd, cur_ua, NULL, &pteshift);
-
-			/* Double check it is still the same pinned page */
-			if (pte && pte_page(*pte) == head &&
-			    pteshift == compshift + PAGE_SHIFT)
-				pageshift = max_t(unsigned int, pteshift,
-						PAGE_SHIFT);
-			local_irq_restore(flags);
+			pageshift = compound_order(head) + PAGE_SHIFT;
 		}
 		mem->pageshift = min(mem->pageshift, pageshift);
 		mem->hpas[i] = page_to_pfn(page) << PAGE_SHIFT;
-- 
2.17.1