powerpc/mm: Differentiate between hugetlb and THP during page walk

We need to properly identify whether a hugepage is an explicit or
a transparent hugepage in follow_huge_addr(). We used to depend
on hugepage shift argument to do that. But in some case that can
result in wrong results. For ex:

On finding a transparent hugepage we set hugepage shift to PMD_SHIFT.
But we can end up clearing the thp pte, via pmdp_huge_get_and_clear.
We do prevent reusing the pfn page via the usage of
kick_all_cpus_sync(). But that happens after we updated the pte to 0.
Hence in follow_huge_addr() we can find hugepage shift set, but transparent
huge page check fail for a thp pte.

NOTE: We fixed a variant of this race against thp split in commit
691e95fd7396905a38d98919e9c150dbc3ea21a3
("powerpc/mm/thp: Make page table walk safe against thp split/collapse")

Without this patch, we may hit the BUG_ON(flags & FOLL_GET) in
follow_page_mask occasionally.

In the long term, we may want to switch ppc64 64k page size config to
enable CONFIG_ARCH_WANT_GENERAL_HUGETLB

Reported-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

3 files changed, 25 insertions, 12 deletions

diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
index aee7017..7f9616f 100644
--- a/arch/powerpc/mm/hash_utils_64.c
+++ b/arch/powerpc/mm/hash_utils_64.c
@@ -994,6 +994,7 @@ int hash_page_mm(struct mm_struct *mm, unsigned long ea,
 		 unsigned long access, unsigned long trap,
 		 unsigned long flags)
 {
+	bool is_thp;
 	enum ctx_state prev_state = exception_enter();
 	pgd_t *pgdir;
 	unsigned long vsid;
@@ -1068,7 +1069,7 @@ int hash_page_mm(struct mm_struct *mm, unsigned long ea,
 #endif /* CONFIG_PPC_64K_PAGES */
 
 	/* Get PTE and page size from page tables */
-	ptep = __find_linux_pte_or_hugepte(pgdir, ea, &hugeshift);
+	ptep = __find_linux_pte_or_hugepte(pgdir, ea, &is_thp, &hugeshift);
 	if (ptep == NULL || !pte_present(*ptep)) {
 		DBG_LOW(" no PTE !\n");
 		rc = 1;
@@ -1088,7 +1089,7 @@ int hash_page_mm(struct mm_struct *mm, unsigned long ea,
 	}
 
 	if (hugeshift) {
-		if (pmd_trans_huge(*(pmd_t *)ptep))
+		if (is_thp)
 			rc = __hash_page_thp(ea, access, vsid, (pmd_t *)ptep,
 					     trap, flags, ssize, psize);
 #ifdef CONFIG_HUGETLB_PAGE
@@ -1243,7 +1244,7 @@ void hash_preload(struct mm_struct *mm, unsigned long ea,
 	 * THP pages use update_mmu_cache_pmd. We don't do
 	 * hash preload there. Hence can ignore THP here
 	 */
-	ptep = find_linux_pte_or_hugepte(pgdir, ea, &hugepage_shift);
+	ptep = find_linux_pte_or_hugepte(pgdir, ea, NULL, &hugepage_shift);
 	if (!ptep)
 		goto out_exit;
 
diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
index f093828..9833fee 100644
--- a/arch/powerpc/mm/hugetlbpage.c
+++ b/arch/powerpc/mm/hugetlbpage.c
@@ -128,7 +128,7 @@ int pgd_huge(pgd_t pgd)
 pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr)
 {
 	/* Only called for hugetlbfs pages, hence can ignore THP */
-	return __find_linux_pte_or_hugepte(mm->pgd, addr, NULL);
+	return __find_linux_pte_or_hugepte(mm->pgd, addr, NULL, NULL);
 }
 
 static int __hugepte_alloc(struct mm_struct *mm, hugepd_t *hpdp,
@@ -703,13 +703,14 @@ void hugetlb_free_pgd_range(struct mmu_gather *tlb,
 struct page *
 follow_huge_addr(struct mm_struct *mm, unsigned long address, int write)
 {
+	bool is_thp;
 	pte_t *ptep, pte;
 	unsigned shift;
 	unsigned long mask, flags;
 	struct page *page = ERR_PTR(-EINVAL);
 
 	local_irq_save(flags);
-	ptep = find_linux_pte_or_hugepte(mm->pgd, address, &shift);
+	ptep = find_linux_pte_or_hugepte(mm->pgd, address, &is_thp, &shift);
 	if (!ptep)
 		goto no_page;
 	pte = READ_ONCE(*ptep);
@@ -718,7 +719,7 @@ follow_huge_addr(struct mm_struct *mm, unsigned long address, int write)
 	 * Transparent hugepages are handled by generic code. We can skip them
 	 * here.
 	 */
-	if (!shift || pmd_trans_huge(__pmd(pte_val(pte))))
+	if (!shift || is_thp)
 		goto no_page;
 
 	if (!pte_present(pte)) {
@@ -975,7 +976,7 @@ void flush_dcache_icache_hugepage(struct page *page)
  */
 
 pte_t *__find_linux_pte_or_hugepte(pgd_t *pgdir, unsigned long ea,
-				   unsigned *shift)
+				   bool *is_thp, unsigned *shift)
 {
 	pgd_t pgd, *pgdp;
 	pud_t pud, *pudp;
@@ -987,6 +988,9 @@ pte_t *__find_linux_pte_or_hugepte(pgd_t *pgdir, unsigned long ea,
 	if (shift)
 		*shift = 0;
 
+	if (is_thp)
+		*is_thp = false;
+
 	pgdp = pgdir + pgd_index(ea);
 	pgd  = READ_ONCE(*pgdp);
 	/*
@@ -1034,7 +1038,14 @@ pte_t *__find_linux_pte_or_hugepte(pgd_t *pgdir, unsigned long ea,
 			if (pmd_none(pmd))
 				return NULL;
 
-			if (pmd_huge(pmd) || pmd_large(pmd)) {
+			if (pmd_trans_huge(pmd)) {
+				if (is_thp)
+					*is_thp = true;
+				ret_pte = (pte_t *) pmdp;
+				goto out;
+			}
+
+			if (pmd_huge(pmd)) {
 				ret_pte = (pte_t *) pmdp;
 				goto out;
 			} else if (is_hugepd(__hugepd(pmd_val(pmd))))
diff --git a/arch/powerpc/mm/tlb_hash64.c b/arch/powerpc/mm/tlb_hash64.c
index c522969..f7b8039 100644
--- a/arch/powerpc/mm/tlb_hash64.c
+++ b/arch/powerpc/mm/tlb_hash64.c
@@ -190,6 +190,7 @@ void tlb_flush(struct mmu_gather *tlb)
 void __flush_hash_table_range(struct mm_struct *mm, unsigned long start,
 			      unsigned long end)
 {
+	bool is_thp;
 	int hugepage_shift;
 	unsigned long flags;
 
@@ -208,21 +209,21 @@ void __flush_hash_table_range(struct mm_struct *mm, unsigned long start,
 	local_irq_save(flags);
 	arch_enter_lazy_mmu_mode();
 	for (; start < end; start += PAGE_SIZE) {
-		pte_t *ptep = find_linux_pte_or_hugepte(mm->pgd, start,
+		pte_t *ptep = find_linux_pte_or_hugepte(mm->pgd, start, &is_thp,
 							&hugepage_shift);
 		unsigned long pte;
 
 		if (ptep == NULL)
 			continue;
 		pte = pte_val(*ptep);
-		if (hugepage_shift)
+		if (is_thp)
 			trace_hugepage_invalidate(start, pte);
 		if (!(pte & _PAGE_HASHPTE))
 			continue;
-		if (unlikely(hugepage_shift && pmd_trans_huge(*(pmd_t *)pte)))
+		if (unlikely(is_thp))
 			hpte_do_hugepage_flush(mm, start, (pmd_t *)ptep, pte);
 		else
-			hpte_need_flush(mm, start, ptep, pte, 0);
+			hpte_need_flush(mm, start, ptep, pte, hugepage_shift);
 	}
 	arch_leave_lazy_mmu_mode();
 	local_irq_restore(flags);