Use ilog2() in fs/namespace.c

We can use ilog2() in fs/namespace.c to compute hash_bits and hash_mask at compile time, not runtime.

[namespace.patch text/plain (1.4KB)]

fs/namespace.c +3 -11

@@ -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);

Quoted 2 lines · On Fri, 14 Dec 2007 18:51:06 +0100 Eric Dumazet <[email protected]> wrote:

We can use ilog2() in fs/namespace.c to compute hash_bits and hash_mask at compile time, not runtime.

Well noted.

Quoted 1 line

[namespace.patch text/plain (1.4KB)]

argh. (save-as, read, copy-paste, s/^/> /g)

Quoted 43 lines

fs/namespace.c +3 -18

@@ -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;
 

fs/namespace.c void __init mnt_init(void) · line 1835

Quoted 1 line

fs/namespace.c +0 -0

 	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:

fs/namespace.c renamed from fs/namespace.c~use-ilog2-in-fs-namespacec-fix +9 -17

@@ -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",

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