[PATCH 6/6] mm: Run the fault-around code under the VMA lock

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

The map_pages fs method should be safe to run under the VMA lock instead
of the mmap lock.  This should have a measurable reduction in contention
on the mmap lock.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/memory.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 9952bebd25b4..590ccaf6f7fb 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4521,8 +4521,6 @@ static vm_fault_t do_read_fault(struct vm_fault *vmf)
 {
 	vm_fault_t ret = 0;
 
-	if (vmf->flags & FAULT_FLAG_VMA_LOCK)
-		return VM_FAULT_RETRY;
 	/*
 	 * Let's call ->map_pages() first and use ->fault() as fallback
 	 * if page by the offset is not ready to be mapped (cold cache or
@@ -4534,6 +4532,8 @@ static vm_fault_t do_read_fault(struct vm_fault *vmf)
 			return ret;
 	}
 
+	if (vmf->flags & FAULT_FLAG_VMA_LOCK)
+		return VM_FAULT_RETRY;
 	ret = __do_fault(vmf);
 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
 		return ret;
-- 
2.39.2