Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932243AbcDNOhh (ORCPT ); Thu, 14 Apr 2016 10:37:37 -0400 Received: from mga03.intel.com ([134.134.136.65]:28466 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932128AbcDNOhc (ORCPT ); Thu, 14 Apr 2016 10:37:32 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.24,485,1455004800"; d="scan'208";a="958683286" From: Matthew Wilcox To: linux-kernel@vger.kernel.org, Andrew Morton Cc: Matthew Wilcox , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Konstantin Khlebnikov , Kirill Shutemov , Jan Kara , Neil Brown , Ross Zwisler Subject: [PATCH 19/19] radix-tree: Free up the bottom bit of exceptional entries for reuse Date: Thu, 14 Apr 2016 10:37:22 -0400 Message-Id: <1460644642-30642-20-git-send-email-willy@linux.intel.com> X-Mailer: git-send-email 2.8.0.rc3 In-Reply-To: <1460644642-30642-1-git-send-email-willy@linux.intel.com> References: <1460644642-30642-1-git-send-email-willy@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3239 Lines: 81 We are guaranteed that pointers to radix_tree_nodes always have the bottom two bits clear (because they come from a slab cache, and slab caches have a minimum alignment of sizeof(void *)), so we can redefine 'radix_tree_is_internal_node' to only return true if the bottom two bits have value '01'. This frees up one quarter of the potential values for use by the user. Idea from Neil Brown Signed-off-by: Matthew Wilcox --- include/linux/radix-tree.h | 38 +++++++++++++++++++++++--------------- 1 file changed, 23 insertions(+), 15 deletions(-) diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h index c2f69e2..cb4b7e8 100644 --- a/include/linux/radix-tree.h +++ b/include/linux/radix-tree.h @@ -29,28 +29,37 @@ #include /* - * Entries in the radix tree have the low bit set if they refer to a - * radix_tree_node. If the low bit is clear then the entry is user data. - * - * We also use the low bit to indicate that the slot will be freed in the - * next RCU idle period, and users need to re-walk the tree to find the - * new slot for the index that they were looking for. See the comment in - * radix_tree_shrink() for details. + * The bottom two bits of the slot determine how the remaining bits in the + * slot are interpreted: + * + * 00 - data pointer + * 01 - internal entry + * 10 - exceptional entry + * 11 - locked exceptional entry + * + * The internal entry may be a pointer to the next level in the tree, a + * sibling entry, or an indicator that the entry in this slot has been moved + * to another location in the tree and the lookup should be restarted. While + * NULL fits the 'data pointer' pattern, it means that there is no entry in + * the tree for this index (no matter what level of the tree it is found at). + * This means that you cannot store NULL in the tree as a value for the index. */ -#define RADIX_TREE_INTERNAL_NODE 1 +#define RADIX_TREE_ENTRY_MASK 3UL +#define RADIX_TREE_INTERNAL_NODE 1UL /* - * A common use of the radix tree is to store pointers to struct pages; - * but shmem/tmpfs needs also to store swap entries in the same tree: - * those are marked as exceptional entries to distinguish them. + * Most users of the radix tree store pointers but shmem/tmpfs stores swap + * entries in the same tree. They are marked as exceptional entries to + * distinguish them from pointers to struct page. * EXCEPTIONAL_ENTRY tests the bit, EXCEPTIONAL_SHIFT shifts content past it. */ #define RADIX_TREE_EXCEPTIONAL_ENTRY 2 #define RADIX_TREE_EXCEPTIONAL_SHIFT 2 -static inline int radix_tree_is_internal_node(void *ptr) +static inline bool radix_tree_is_internal_node(void *ptr) { - return (int)((unsigned long)ptr & RADIX_TREE_INTERNAL_NODE); + return ((unsigned long)ptr & RADIX_TREE_ENTRY_MASK) == + RADIX_TREE_INTERNAL_NODE; } /*** radix-tree API starts here ***/ @@ -236,8 +245,7 @@ static inline int radix_tree_exceptional_entry(void *arg) */ static inline int radix_tree_exception(void *arg) { - return unlikely((unsigned long)arg & - (RADIX_TREE_INTERNAL_NODE | RADIX_TREE_EXCEPTIONAL_ENTRY)); + return unlikely((unsigned long)arg & RADIX_TREE_ENTRY_MASK); } /** -- 2.8.0.rc3