powerpc/mm/thp: Make page table walk safe against thp split/collapse

We can disable a THP split or a hugepage collapse by disabling irq.
We do send IPI to all the cpus in the early part of split/collapse,
and disabling local irq ensure we don't make progress with
split/collapse. If the THP is getting split we return NULL from
find_linux_pte_or_hugepte(). For all the current callers it should be ok.
We need to be careful if we want to use returned pte_t pointer outside
the irq disabled region. W.r.t to THP split, the pfn remains the same,
but then a hugepage collapse will result in a pfn change. There are
few steps we can take to avoid a hugepage collapse.One way is to take page
reference inside the irq disable region. Other option is to take
mmap_sem so that a parallel collapse will not happen. We can also
disable collapse by taking pmd_lock. Another method used by kvm
subsystem is to check whether we had a mmu_notifer update in between
using mmu_notifier_retry().

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

1 files changed, 23 insertions, 7 deletions

diff --git a/arch/powerpc/kvm/book3s_hv_rm_mmu.c b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
index 73e083cb9f7e..f559b25de173 100644
--- a/arch/powerpc/kvm/book3s_hv_rm_mmu.c
+++ b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
@@ -26,11 +26,14 @@ static void *real_vmalloc_addr(void *x)
 {
 	unsigned long addr = (unsigned long) x;
 	pte_t *p;
-
-	p = find_linux_pte_or_hugepte(swapper_pg_dir, addr, NULL);
+	/*
+	 * assume we don't have huge pages in vmalloc space...
+	 * So don't worry about THP collapse/split. Called
+	 * Only in realmode, hence won't need irq_save/restore.
+	 */
+	p = __find_linux_pte_or_hugepte(swapper_pg_dir, addr, NULL);
 	if (!p || !pte_present(*p))
 		return NULL;
-	/* assume we don't have huge pages in vmalloc space... */
 	addr = (pte_pfn(*p) << PAGE_SHIFT) | (addr & ~PAGE_MASK);
 	return __va(addr);
 }
@@ -153,7 +156,7 @@ long kvmppc_do_h_enter(struct kvm *kvm, unsigned long flags,
 	pte_t *ptep;
 	unsigned int writing;
 	unsigned long mmu_seq;
-	unsigned long rcbits;
+	unsigned long rcbits, irq_flags = 0;
 
 	psize = hpte_page_size(pteh, ptel);
 	if (!psize)
@@ -189,7 +192,16 @@ long kvmppc_do_h_enter(struct kvm *kvm, unsigned long flags,
 
 	/* Translate to host virtual address */
 	hva = __gfn_to_hva_memslot(memslot, gfn);
-	ptep = find_linux_pte_or_hugepte(pgdir, hva, &hpage_shift);
+	/*
+	 * If we had a page table table change after lookup, we would
+	 * retry via mmu_notifier_retry.
+	 */
+	if (realmode)
+		ptep = __find_linux_pte_or_hugepte(pgdir, hva, &hpage_shift);
+	else {
+		local_irq_save(irq_flags);
+		ptep = find_linux_pte_or_hugepte(pgdir, hva, &hpage_shift);
+	}
 	if (ptep) {
 		pte_t pte;
 		unsigned int host_pte_size;
@@ -202,9 +214,11 @@ long kvmppc_do_h_enter(struct kvm *kvm, unsigned long flags,
 		 * We should always find the guest page size
 		 * to <= host page size, if host is using hugepage
 		 */
-		if (host_pte_size < psize)
+		if (host_pte_size < psize) {
+			if (!realmode)
+				local_irq_restore(flags);
 			return H_PARAMETER;
-
+		}
 		pte = kvmppc_read_update_linux_pte(ptep, writing, hpage_shift);
 		if (pte_present(pte) && !pte_protnone(pte)) {
 			if (writing && !pte_write(pte))
@@ -216,6 +230,8 @@ long kvmppc_do_h_enter(struct kvm *kvm, unsigned long flags,
 			pa |= gpa & ~PAGE_MASK;
 		}
 	}
+	if (!realmode)
+		local_irq_restore(irq_flags);
 
 	ptel &= ~(HPTE_R_PP0 - psize);
 	ptel |= pa;