mm: do_wp_page() simplification

How about we just make sure we're the only possible valid user fo the
page before we bother to reuse it?

Simplify, simplify, simplify.

And get rid of the nasty serialization on the page lock at the same time.

[peterx: add subject prefix]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -2924,50 +2924,25 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 	 * not dirty accountable.
 	 */
 	if (PageAnon(vmf->page)) {
-		int total_map_swapcount;
-		if (PageKsm(vmf->page) && (PageSwapCache(vmf->page) ||
-					   page_count(vmf->page) != 1))
+		struct page *page = vmf->page;
+
+		/* PageKsm() doesn't necessarily raise the page refcount */
+		if (PageKsm(page) || page_count(page) != 1)
 			goto copy;
-		if (!trylock_page(vmf->page)) {
-			get_page(vmf->page);
-			pte_unmap_unlock(vmf->pte, vmf->ptl);
-			lock_page(vmf->page);
-			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
-					vmf->address, &vmf->ptl);
-			if (!pte_same(*vmf->pte, vmf->orig_pte)) {
-				update_mmu_tlb(vma, vmf->address, vmf->pte);
-				unlock_page(vmf->page);
-				pte_unmap_unlock(vmf->pte, vmf->ptl);
-				put_page(vmf->page);
-				return 0;
-			}
-			put_page(vmf->page);
-		}
-		if (PageKsm(vmf->page)) {
-			bool reused = reuse_ksm_page(vmf->page, vmf->vma,
-						     vmf->address);
-			unlock_page(vmf->page);
-			if (!reused)
+		if (!trylock_page(page))
+			goto copy;
+		if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) {
+			unlock_page(page);
 			goto copy;
-			wp_page_reuse(vmf);
-			return VM_FAULT_WRITE;
 		}
-		if (reuse_swap_page(vmf->page, &total_map_swapcount)) {
-			if (total_map_swapcount == 1) {
 		/*
-				 * The page is all ours. Move it to
-				 * our anon_vma so the rmap code will
-				 * not search our parent or siblings.
-				 * Protected against the rmap code by
-				 * the page lock.
+		 * Ok, we've got the only map reference, and the only
+		 * page count reference, and the page is locked,
+		 * it's dark out, and we're wearing sunglasses. Hit it.
 		 */
-				page_move_anon_rmap(vmf->page, vma);
-			}
-			unlock_page(vmf->page);
 		wp_page_reuse(vmf);
+		unlock_page(page);
 		return VM_FAULT_WRITE;
-		}
-		unlock_page(vmf->page);
 	} else if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
 					(VM_WRITE|VM_SHARED))) {
 		return wp_page_shared(vmf);