From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with ESMTP id 0BDC68D0039 for ; Thu, 24 Feb 2011 00:32:02 -0500 (EST) Received: from wpaz17.hot.corp.google.com (wpaz17.hot.corp.google.com [172.24.198.81]) by smtp-out.google.com with ESMTP id p1O5W07W020662 for ; Wed, 23 Feb 2011 21:32:00 -0800 Received: from gwb15 (gwb15.prod.google.com [10.200.2.15]) by wpaz17.hot.corp.google.com with ESMTP id p1O5Vx5L009471 (version=TLSv1/SSLv3 cipher=RC4-SHA bits=128 verify=NOT) for ; Wed, 23 Feb 2011 21:31:59 -0800 Received: by gwb15 with SMTP id 15so166538gwb.10 for ; Wed, 23 Feb 2011 21:31:59 -0800 (PST) Date: Wed, 23 Feb 2011 21:31:50 -0800 (PST) From: Hugh Dickins Subject: [PATCH] mm: remove worrying dead code from find_get_pages() Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Nick Piggin Cc: Andrew Morton , Peter Zijlstra , Wu Fengguang , Salman Qazi , linux-kernel@vger.kernel.org, linux-mm@kvack.org 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 --- 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; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org