mm: remove worrying dead code from find_get_pages()

The radix_tree_deref_retry() case in find_get_pages() has a strange little
excrescence, not seen in the other gang lookups: it looks like the start
of an abandoned attempt to guarantee forward progress in a case that
cannot arise.

ret should always be 0 here: if it isn't, then going back to restart will
leak references to pages already gotten.  There used to be a comment
saying nr_found is necessarily 1 here: that's not quite true, but the
radix_tree_deref_retry() case is peculiar to the entry at index 0, when we
race with it being moved out of the radix_tree root or back.

Remove the worrisome two lines, add a brief comment here and in
find_get_pages_contig() and find_get_pages_tag(), and a WARN_ON in
find_get_pages() should it ever be seen elsewhere than at 0.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Nick Piggin <npiggin@kernel.dk>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Salman Qazi <sqazi@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 2 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index cb476e70cf19..a29318147365 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -863,9 +863,13 @@ repeat:
                 page = radix_tree_deref_slot((void **)pages[i]);
                 if (unlikely(!page))
                         continue;
+
+                /*
+                 * This can only trigger when the entry at index 0 moves out
+                 * of or back to the root: none yet gotten, safe to restart.
+                 */
                 if (radix_tree_deref_retry(page)) {
-                        if (ret)
-                                start = pages[ret-1]->index;
+                        WARN_ON(start | i);
                         goto restart;
                 }
 
@@ -915,6 +919,11 @@ repeat:
                 page = radix_tree_deref_slot((void **)pages[i]);
                 if (unlikely(!page))
                         continue;
+
+                /*
+                 * This can only trigger when the entry at index 0 moves out
+                 * of or back to the root: none yet gotten, safe to restart.
+                 */
                 if (radix_tree_deref_retry(page))
                         goto restart;
 
@@ -975,6 +984,11 @@ repeat:
                 page = radix_tree_deref_slot((void **)pages[i]);
                 if (unlikely(!page))
                         continue;
+
+                /*
+                 * This can only trigger when the entry at index 0 moves out
+                 * of or back to the root: none yet gotten, safe to restart.
+                 */
                 if (radix_tree_deref_retry(page))
                         goto restart;