linux-kernel - [PATCH] mm: remove worrying dead code from find_get

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Date:	Wed, 23 Feb 2011 21:31:50 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Nick Piggin <npiggin@...nel.dk>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Salman Qazi <sqazi@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: [PATCH] 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@...gle.com>
---

 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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