From: Janosch Frank <frankja@linux.ibm.com>
Subject: KVM: s390: Properly lock mm context allow_gmap_hpage_1m setting
Patch-mainline: v4.19-rc3
Git-commit: df88f3181f10565c6e3a89eb6f0f9e6afaaf15f1
References: FATE#326372, LTC#169184, bsc#1113484

Summary:     kernel: Introduce huge page KVM backing support
Description: This adds the KVM support for libhugetlbfs backings of
             s390 KVM guests.

Upstream-Description:

             KVM: s390: Properly lock mm context allow_gmap_hpage_1m setting

             We have to do down_write on the mm semaphore to set a bitfield in the
             mm context.

             Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
             Fixes: a4499382 ("KVM: s390: Add huge page enablement control")
             Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
             Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Acked-by: Petr Tesarik <ptesarik@suse.com>
---
 arch/s390/include/asm/mmu.h |    8 +++++++-
 arch/s390/kvm/kvm-s390.c    |    2 ++
 2 files changed, 9 insertions(+), 1 deletion(-)

--- a/arch/s390/include/asm/mmu.h
+++ b/arch/s390/include/asm/mmu.h
@@ -17,7 +17,13 @@ typedef struct {
 	unsigned long asce;
 	unsigned long asce_limit;
 	unsigned long vdso_base;
-	/* The mmu context allocates 4K page tables. */
+	/*
+	 * The following bitfields need a down_write on the mm
+	 * semaphore when they are written to. As they are only
+	 * written once, they can be read without a lock.
+	 *
+	 * The mmu context allocates 4K page tables.
+	 */
 	unsigned int alloc_pgste:1;
 	/* The mmu context uses extended page tables. */
 	unsigned int has_pgste:1;
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -674,7 +674,9 @@ static int kvm_vm_ioctl_enable_cap(struc
 			r = -EINVAL;
 		else {
 			r = 0;
+			down_write(&kvm->mm->mmap_sem);
 			kvm->mm->context.allow_gmap_hpage_1m = 1;
+			up_write(&kvm->mm->mmap_sem);
 			/*
 			 * We might have to create fake 4k page
 			 * tables. To avoid that the hardware works on