The patch titled
mm: remove worrying dead code from find_get_pages()
has been added to the -mm tree. Its filename is
mm-remove-worrying-dead-code-from-find_get_pages.patch
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out what to do about this
The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
------------------------------------------------------
Subject: mm: remove worrying dead code from find_get_pages()
From: Hugh Dickins <hughd@xxxxxxxxxx>
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>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Wu Fengguang <fengguang.wu@xxxxxxxxx>
Cc: Salman Qazi <sqazi@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
diff -puN mm/filemap.c~mm-remove-worrying-dead-code-from-find_get_pages mm/filemap.c
--- a/mm/filemap.c~mm-remove-worrying-dead-code-from-find_get_pages
+++ a/mm/filemap.c
@@ -875,9 +875,13 @@ 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;
}
@@ -927,6 +931,11 @@ 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;
@@ -987,6 +996,11 @@ 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;
_
Patches currently in -mm which might be from hughd@xxxxxxxxxx are
origin.patch
linux-next.patch
mm-vmap-area-cache.patch
mm-allow-gup-to-fail-instead-of-waiting-on-a-page.patch
mm-allow-gup-to-fail-instead-of-waiting-on-a-page-fix.patch
mm-introduce-delete_from_page_cache.patch
mm-hugetlbfs-change-remove_from_page_cache.patch
mm-shmem-change-remove_from_page_cache.patch
mm-truncate-change-remove_from_page_cache.patch
mm-good-bye-remove_from_page_cache.patch
mm-change-__remove_from_page_cache.patch
mm-rename-drop_anon_vma-to-put_anon_vma.patch
mm-move-anon_vma-ref-out-from-under-config_foo.patch
mm-simplify-anon_vma-refcounts.patch
mm-remove-worrying-dead-code-from-find_get_pages.patch
prio_tree-debugging-patch.patch
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