[PATCH 2/2] s390/mm: enable fixup_user_fault retrying

[Dominik Dingel]

By passing a non-null flag we allow fixup_user_fault to retry, which
enables userfaultfd.  As during these retries we might drop the mmap_sem we
need to check if that happened and redo the complete chain of actions.

Signed-off-by: Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx>
---
 arch/s390/mm/pgtable.c | 29 ++++++++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
index b15759c..3c5456d 100644
--- a/arch/s390/mm/pgtable.c
+++ b/arch/s390/mm/pgtable.c
@@ -578,17 +578,29 @@ int gmap_fault(struct gmap *gmap, unsigned long gaddr,
 {
 	unsigned long vmaddr;
 	int rc;
+	bool unlocked;
 
 	down_read(&gmap->mm->mmap_sem);
+
+retry:
+	unlocked = false;
 	vmaddr = __gmap_translate(gmap, gaddr);
 	if (IS_ERR_VALUE(vmaddr)) {
 		rc = vmaddr;
 		goto out_up;
 	}
-	if (fixup_user_fault(current, gmap->mm, vmaddr, fault_flags, NULL)) {
+	if (fixup_user_fault(current, gmap->mm, vmaddr, fault_flags,
+			     &unlocked)) {
 		rc = -EFAULT;
 		goto out_up;
 	}
+	/*
+	 * In the case that fixup_user_fault unlocked the mmap_sem during
+	 * faultin redo __gmap_translate to not race with a map/unmap_segment.
+	 */
+	if (unlocked)
+		goto retry;
+
 	rc = __gmap_link(gmap, gaddr, vmaddr);
 out_up:
 	up_read(&gmap->mm->mmap_sem);
@@ -717,12 +729,14 @@ int gmap_ipte_notify(struct gmap *gmap, unsigned long gaddr, unsigned long len)
 	spinlock_t *ptl;
 	pte_t *ptep, entry;
 	pgste_t pgste;
+	bool unlocked;
 	int rc = 0;
 
 	if ((gaddr & ~PAGE_MASK) || (len & ~PAGE_MASK))
 		return -EINVAL;
 	down_read(&gmap->mm->mmap_sem);
 	while (len) {
+		unlocked = false;
 		/* Convert gmap address and connect the page tables */
 		addr = __gmap_translate(gmap, gaddr);
 		if (IS_ERR_VALUE(addr)) {
@@ -731,10 +745,13 @@ int gmap_ipte_notify(struct gmap *gmap, unsigned long gaddr, unsigned long len)
 		}
 		/* Get the page mapped */
 		if (fixup_user_fault(current, gmap->mm, addr, FAULT_FLAG_WRITE,
-				     NULL)) {
+				     &unlocked)) {
 			rc = -EFAULT;
 			break;
 		}
+		/* While trying to map mmap_sem got unlocked. Let us retry */
+		if (unlocked)
+			continue;
 		rc = __gmap_link(gmap, gaddr, addr);
 		if (rc)
 			break;
@@ -795,9 +812,11 @@ int set_guest_storage_key(struct mm_struct *mm, unsigned long addr,
 	spinlock_t *ptl;
 	pgste_t old, new;
 	pte_t *ptep;
+	bool unlocked;
 
 	down_read(&mm->mmap_sem);
 retry:
+	unlocked = false;
 	ptep = get_locked_pte(mm, addr, &ptl);
 	if (unlikely(!ptep)) {
 		up_read(&mm->mmap_sem);
@@ -806,8 +825,12 @@ retry:
 	if (!(pte_val(*ptep) & _PAGE_INVALID) &&
 	     (pte_val(*ptep) & _PAGE_PROTECT)) {
 		pte_unmap_unlock(ptep, ptl);
+		/*
+		 * We do not really care about unlocked. We will retry either
+		 * way. But this allows fixup_user_fault to enable userfaultfd.
+		 */
 		if (fixup_user_fault(current, mm, addr, FAULT_FLAG_WRITE,
-				     NULL)) {
+				     &unlocked)) {
 			up_read(&mm->mmap_sem);
 			return -EFAULT;
 		}
-- 
2.3.9

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