mm: improve find_lock_page

find_lock_page does not need to recheck ->index because if the page is in the
right mapping then the index must be the same.  Also, tree_lock does not need
to be retaken after the page is locked in order to test that ->mapping has not
changed, because holding the page lock pins its mapping.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

diff --git a/mm/filemap.c b/mm/filemap.c
index b436cbb..7989c44 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -621,26 +621,27 @@
 {
 	struct page *page;
 
-	read_lock_irq(&mapping->tree_lock);
 repeat:
+	read_lock_irq(&mapping->tree_lock);
 	page = radix_tree_lookup(&mapping->page_tree, offset);
 	if (page) {
 		page_cache_get(page);
 		if (TestSetPageLocked(page)) {
 			read_unlock_irq(&mapping->tree_lock);
 			__lock_page(page);
-			read_lock_irq(&mapping->tree_lock);
 
 			/* Has the page been truncated while we slept? */
-			if (unlikely(page->mapping != mapping ||
-				     page->index != offset)) {
+			if (unlikely(page->mapping != mapping)) {
 				unlock_page(page);
 				page_cache_release(page);
 				goto repeat;
 			}
+			VM_BUG_ON(page->index != offset);
+			goto out;
 		}
 	}
 	read_unlock_irq(&mapping->tree_lock);
+out:
 	return page;
 }
 EXPORT_SYMBOL(find_lock_page);