[RFC PATCH v4 08/13] vfio: remove unnecessary mmap_sem writer acquisition around locked_vm

[Daniel Jordan <daniel.m.jordan@oracle.com>]

Now that mmap_sem is no longer required for modifying locked_vm, remove
it in the VFIO code.

[XXX Can be sent separately, along with similar conversions in the other
places mmap_sem was taken for locked_vm.  While at it, could make
similar changes to pinned_vm.]

Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
---
 drivers/vfio/vfio_iommu_type1.c | 26 +++++++++-----------------
 1 file changed, 9 insertions(+), 17 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index f307dc9d5e19..9e52a24eb45b 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -258,7 +258,8 @@ static int vfio_iova_put_vfio_pfn(struct vfio_dma *dma, struct vfio_pfn *vpfn)
 static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
 {
 	struct mm_struct *mm;
-	int ret;
+	long locked_vm;
+	int ret = 0;
 
 	if (!npage)
 		return 0;
@@ -267,24 +268,15 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
 	if (!mm)
 		return -ESRCH; /* process exited */
 
-	ret = down_write_killable(&mm->mmap_sem);
-	if (!ret) {
-		if (npage > 0) {
-			if (!dma->lock_cap) {
-				unsigned long limit;
-
-				limit = task_rlimit(dma->task,
-						RLIMIT_MEMLOCK) >> PAGE_SHIFT;
+	locked_vm = atomic_long_add_return(npage, &mm->locked_vm);
 
-				if (atomic_long_read(&mm->locked_vm) + npage > limit)
-					ret = -ENOMEM;
-			}
+	if (npage > 0 && !dma->lock_cap) {
+		unsigned long limit = task_rlimit(dma->task, RLIMIT_MEMLOCK) >>
+					  PAGE_SHIFT;
+		if (locked_vm > limit) {
+			atomic_long_sub(npage, &mm->locked_vm);
+			ret = -ENOMEM;
 		}
-
-		if (!ret)
-			atomic_long_add(npage, &mm->locked_vm);
-
-		up_write(&mm->mmap_sem);
 	}
 
 	if (async)
-- 
2.19.1