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@xxxxxxxxxx>
---
mm/filemap.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
--- 2.6.38-rc6/mm/filemap.c 2011-01-18 22:04:56.000000000 -0800
+++ linux/mm/filemap.c 2011-02-23 15:45:47.000000000 -0800
@@ -782,9 +782,13 @@ 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;
}
@@ -834,6 +838,11 @@ 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;
@@ -894,6 +903,11 @@ 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;
--
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