Subject: [PATCH V4 3/3] powerpc/mm/iommu: Allow large IOMMU page size only for hugetlb backing

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Wed, 21 Nov 2018 14:52:59 +0530

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 <[email-masked]>
---
 arch/powerpc/mm/mmu_context_iommu.c | 24 +++++++-----------------
 1 file changed, 7 insertions(+), 17 deletions(-)

diff --git a/arch/powerpc/mm/mmu_context_iommu.c b/arch/powerpc/mm/mmu_context_iommu.c
index 1d5161f93ce6..0741d905ed04 100644
--- a/arch/powerpc/mm/mmu_context_iommu.c
+++ b/arch/powerpc/mm/mmu_context_iommu.c
@@ -95,8 +95,6 @@ long mm_iommu_get(struct mm_struct *mm, unsigned long ua, unsigned long entries,
 	struct mm_iommu_table_group_mem_t *mem;
 	long i, ret = 0, locked_entries = 0;
 	unsigned int pageshift;
-	unsigned long flags;
-	unsigned long cur_ua;
 
 	mutex_lock(&mem_list_mutex);
 
@@ -159,23 +157,15 @@ long mm_iommu_get(struct mm_struct *mm, unsigned long ua, unsigned long entries,
 	pageshift = PAGE_SHIFT;
 	for (i = 0; i < entries; ++i) {
 		struct page *page = mem->hpages[i];
-		cur_ua = ua + (i << PAGE_SHIFT);
 
-		if (mem->pageshift > PAGE_SHIFT && PageCompound(page)) {
-			pte_t *pte;
+		/*
+		 * Allow to use larger than 64k IOMMU pages. Only do that
+		 * if we are backed by hugetlb.
+		 */
+		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);
 		/*
-- 
2.17.2