mm: handle faults that merely update the accessed bit under the VMA lock

Move FAULT_FLAG_VMA_LOCK check out of handle_pte_fault().  This should
have a significant performance improvement for mmaped files.  Write faults
(on read-only shared pages) still take the mmap lock as we do not want to
audit all the implementations of ->pfn_mkwrite() and ->page_mkwrite(). 
However write-faults on private mappings are handled under the VMA lock.

[willy@infradead.org: address "suspicious RCU usage" warning]
  Link: https://lkml.kernel.org/r/ZMK7jwpI4uD6tKrF@casper.infradead.org
Link: https://lkml.kernel.org/r/20230724185410.1124082-11-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Arjun Roy <arjunroy@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Punit Agrawal <punit.agrawal@bytedance.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memory.c

@@ -3268,6 +3268,11 @@ static vm_fault_t wp_pfn_shared(struct vm_fault *vmf)
 		vm_fault_t ret;
 
 		pte_unmap_unlock(vmf->pte, vmf->ptl);
+		if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
+			vma_end_read(vmf->vma);
+			return VM_FAULT_RETRY;
+		}
+
 		vmf->flags |= FAULT_FLAG_MKWRITE;
 		ret = vma->vm_ops->pfn_mkwrite(vmf);
 		if (ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))
@@ -3290,6 +3295,12 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf, struct folio *folio)
 		vm_fault_t tmp;
 
 		pte_unmap_unlock(vmf->pte, vmf->ptl);
+		if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
+			folio_put(folio);
+			vma_end_read(vmf->vma);
+			return VM_FAULT_RETRY;
+		}
+
 		tmp = do_page_mkwrite(vmf, folio);
 		if (unlikely(!tmp || (tmp &
 				      (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
@@ -3431,6 +3442,12 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 		return 0;
 	}
 copy:
+	if ((vmf->flags & FAULT_FLAG_VMA_LOCK) && !vma->anon_vma) {
+		pte_unmap_unlock(vmf->pte, vmf->ptl);
+		vma_end_read(vmf->vma);
+		return VM_FAULT_RETRY;
+	}
+
 	/*
 	 * Ok, we need to copy. Oh, well..
 	 */
@@ -4985,12 +5002,6 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
 	if (pte_protnone(vmf->orig_pte) && vma_is_accessible(vmf->vma))
 		return do_numa_page(vmf);
 
-	if ((vmf->flags & FAULT_FLAG_VMA_LOCK) && !vma_is_anonymous(vmf->vma)) {
-		pte_unmap(vmf->pte);
-		vma_end_read(vmf->vma);
-		return VM_FAULT_RETRY;
-	}
-
 	spin_lock(vmf->ptl);
 	entry = vmf->orig_pte;
 	if (unlikely(!pte_same(ptep_get(vmf->pte), entry))) {