mm: shmem: save one radix tree lookup when truncating swapped pages

Page cache radix tree slots are usually stabilized by the page lock, but
shmem's swap cookies have no such thing.  Because the overall truncation
loop is lockless, the swap entry is currently confirmed by a tree lookup
and then deleted by another tree lookup under the same tree lock region.

Use radix_tree_delete_item() instead, which does the verification and
deletion with only one lookup.  This also allows removing the
delete-only special case from shmem_radix_tree_replace().

Signed-off-by: Johannes Weiner <[email protected]>
Reviewed-by: Minchan Kim <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Bob Liu <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Cc: Luigi Semenzato <[email protected]>
Cc: Metin Doslu <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Ozgun Erdogan <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Ryan Mallon <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: hnaz

mm/shmem.c

@@ -242,19 +242,17 @@ static int shmem_radix_tree_replace(struct address_space *mapping,
 			pgoff_t index, void *expected, void *replacement)
 {
 	void **pslot;
-	void *item = NULL;
+	void *item;
 
 	VM_BUG_ON(!expected);
+	VM_BUG_ON(!replacement);
 	pslot = radix_tree_lookup_slot(&mapping->page_tree, index);
-	if (pslot)
-		item = radix_tree_deref_slot_protected(pslot,
-							&mapping->tree_lock);
+	if (!pslot)
+		return -ENOENT;
+	item = radix_tree_deref_slot_protected(pslot, &mapping->tree_lock);
 	if (item != expected)
 		return -ENOENT;
-	if (replacement)
-		radix_tree_replace_slot(pslot, replacement);
-	else
-		radix_tree_delete(&mapping->page_tree, index);
+	radix_tree_replace_slot(pslot, replacement);
 	return 0;
 }
 
@@ -386,14 +384,15 @@ static unsigned shmem_find_get_pages_and_swap(struct address_space *mapping,
 static int shmem_free_swap(struct address_space *mapping,
 			   pgoff_t index, void *radswap)
 {
-	int error;
+	void *old;
 
 	spin_lock_irq(&mapping->tree_lock);
-	error = shmem_radix_tree_replace(mapping, index, radswap, NULL);
+	old = radix_tree_delete_item(&mapping->page_tree, index, radswap);
 	spin_unlock_irq(&mapping->tree_lock);
-	if (!error)
-		free_swap_and_cache(radix_to_swp_entry(radswap));
-	return error;
+	if (old != radswap)
+		return -ENOENT;
+	free_swap_and_cache(radix_to_swp_entry(radswap));
+	return 0;
 }
 
 /*