Re: [PATCH] Use ilog2() in fs/namespace.c

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 14 Dec 2007 18:51:06 +0100 Eric Dumazet <dada1@cosmosbay.com> 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 <linux/security.h>
>  #include <linux/mount.h>
>  #include <linux/ramfs.h>
> +#include <linux/log2.h>
>  #include <asm/uaccess.h>
>  #include <asm/unistd.h>
>  #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",
_