From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756509AbXLUKaI (ORCPT ); Fri, 21 Dec 2007 05:30:08 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751321AbXLUK34 (ORCPT ); Fri, 21 Dec 2007 05:29:56 -0500 Received: from smtp2.linux-foundation.org ([207.189.120.14]:33608 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751091AbXLUK3z (ORCPT ); Fri, 21 Dec 2007 05:29:55 -0500 Date: Fri, 21 Dec 2007 02:29:12 -0800 From: Andrew Morton To: Eric Dumazet Cc: linux kernel , Al Viro Subject: Re: [PATCH] Use ilog2() in fs/namespace.c Message-Id: <20071221022912.72b6274e.akpm@linux-foundation.org> In-Reply-To: <4762C28A.4040002@cosmosbay.com> References: <4762C28A.4040002@cosmosbay.com> X-Mailer: Sylpheed 2.4.1 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 14 Dec 2007 18:51:06 +0100 Eric Dumazet wrote: > We can use ilog2() in fs/namespace.c to compute hash_bits and hash_mask at > compile time, not runtime. Well noted. > [namespace.patch text/plain (1.4KB)] argh. (save-as, read, copy-paste, s/^/> /g) > --- a/fs/namespace.c > +++ b/fs/namespace.c > @@ -25,6 +25,7 @@ > #include > #include > #include > +#include > #include > #include > #include "pnode.h" > @@ -36,7 +37,8 @@ __cacheline_aligned_in_smp DEFINE_SPINLOCK(vfsmount_lock); > static int event; > > static struct list_head *mount_hashtable __read_mostly; > -static int hash_mask __read_mostly, hash_bits __read_mostly; > +#define hash_bits ilog2(PAGE_SIZE / sizeof(struct list_head)) > +#define hash_mask ((1UL << hash_bits) - 1) > static struct kmem_cache *mnt_cache __read_mostly; > static struct rw_semaphore namespace_sem; > > @@ -1828,24 +1830,7 @@ void __init mnt_init(void) > if (!mount_hashtable) > panic("Failed to allocate mount hash table\n"); > > - /* > - * Find the power-of-two list-heads that can fit into the allocation.. > - * We don't guarantee that "sizeof(struct list_head)" is necessarily > - * a power-of-two. > - */ > - nr_hash = PAGE_SIZE / sizeof(struct list_head); > - hash_bits = 0; > - do { > - hash_bits++; > - } while ((nr_hash >> hash_bits) != 0); > - hash_bits--; > - > - /* > - * Re-calculate the actual number of entries and the mask > - * from the number of bits we can fit. > - */ > nr_hash = 1UL << hash_bits; > - hash_mask = nr_hash - 1; > > printk("Mount-cache hash table entries: %d\n", nr_hash); Those #defines you now have there are foul. Please, when there's a choice between doing it minimally and doing it right, let's do it right? Look what we can now do: --- a/fs/namespace.c~use-ilog2-in-fs-namespacec-fix +++ a/fs/namespace.c @@ -31,14 +31,15 @@ #include "pnode.h" #include "internal.h" +#define HASH_SHIFT ilog2(PAGE_SIZE / sizeof(struct list_head)) +#define HASH_SIZE (1UL << HASH_SHIFT) + /* spinlock for vfsmount related operations, inplace of dcache_lock */ __cacheline_aligned_in_smp DEFINE_SPINLOCK(vfsmount_lock); static int event; static struct list_head *mount_hashtable __read_mostly; -#define hash_bits ilog2(PAGE_SIZE / sizeof(struct list_head)) -#define hash_mask ((1UL << hash_bits) - 1) static struct kmem_cache *mnt_cache __read_mostly; static struct rw_semaphore namespace_sem; @@ -50,8 +51,8 @@ static inline unsigned long hash(struct { unsigned long tmp = ((unsigned long)mnt / L1_CACHE_BYTES); tmp += ((unsigned long)dentry / L1_CACHE_BYTES); - tmp = tmp + (tmp >> hash_bits); - return tmp & hash_mask; + tmp = tmp + (tmp >> HASH_SHIFT); + return tmp & (HASH_SIZE - 1); } struct vfsmount *alloc_vfsmnt(const char *name) @@ -1815,9 +1816,7 @@ static void __init init_mount_tree(void) void __init mnt_init(void) { - struct list_head *d; - unsigned int nr_hash; - int i; + unsigned u; int err; init_rwsem(&namespace_sem); @@ -1830,18 +1829,11 @@ void __init mnt_init(void) if (!mount_hashtable) panic("Failed to allocate mount hash table\n"); - nr_hash = 1UL << hash_bits; + printk("Mount-cache hash table entries: %lu\n", HASH_SIZE); - printk("Mount-cache hash table entries: %d\n", nr_hash); + for (u = 0; u < HASH_SIZE; u++) + INIT_LIST_HEAD(&mount_hashtable[u]); - /* And initialize the newly allocated array */ - d = mount_hashtable; - i = nr_hash; - do { - INIT_LIST_HEAD(d); - d++; - i--; - } while (i); err = sysfs_init(); if (err) printk(KERN_WARNING "%s: sysfs_init error: %d\n", _