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>

1 files changed, 5 insertions, 4 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index b436cbb3a834..7989c44cb293 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -621,26 +621,27 @@ struct page *find_lock_page(struct address_space *mapping,
 {
         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);