mm: run the fault-around code under the VMA lock

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.

Link: https://lkml.kernel.org/r/20230724185410.1124082-9-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Arjun Roy <arjunroy@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Punit Agrawal <punit.agrawal@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memory.c

@@ -4533,11 +4533,6 @@ static vm_fault_t do_read_fault(struct vm_fault *vmf)
 	vm_fault_t ret = 0;
 	struct folio *folio;
 
-	if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
-		vma_end_read(vmf->vma);
-		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
@@ -4549,6 +4544,11 @@ static vm_fault_t do_read_fault(struct vm_fault *vmf)
 			return ret;
 	}
 
+	if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
+		vma_end_read(vmf->vma);
+		return VM_FAULT_RETRY;
+	}
+
 	ret = __do_fault(vmf);
 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
 		return ret;