Re: [PATCH 02/16] user_ns: use new hashtable implementation
From: Eric W. Biederman <hidden>
Date: 2012-08-14 23:52:28
Sasha Levin [off-list ref] writes:
Switch user_ns to use the new hashtable implementation. This reduces the amount of generic unrelated code in user_ns.
Two concerns here. 1) When adding a new entry you recompute the hash where previously that was not done. I believe that will slow down adding of new entries. 2) Using hash_32 for uids is an interesting choice. hash_32 discards the low bits. Last I checked for uids the low bits were the bits that were most likely to be different and had the most entropy. I'm not certain how multiplying by the GOLDEN_RATION_PRIME_32 will affect things but I would be surprised if it shifted all of the randomness from the low bits to the high bits. And just a nit. struct user is essentially orthogonal to the user namespace at this point, making the description of the patch a little weird. Eric
quoted hunk
Signed-off-by: Sasha Levin <redacted> --- kernel/user.c | 33 +++++++++++++-------------------- 1 files changed, 13 insertions(+), 20 deletions(-)diff --git a/kernel/user.c b/kernel/user.c index b815fef..d10c484 100644 --- a/kernel/user.c +++ b/kernel/user.c@@ -16,6 +16,7 @@ #include <linux/interrupt.h> #include <linux/export.h> #include <linux/user_namespace.h> +#include <linux/hashtable.h> /* * userns count is 1 for root user, 1 for init_uts_ns,@@ -52,13 +53,9 @@ EXPORT_SYMBOL_GPL(init_user_ns); */ #define UIDHASH_BITS (CONFIG_BASE_SMALL ? 3 : 7) -#define UIDHASH_SZ (1 << UIDHASH_BITS) -#define UIDHASH_MASK (UIDHASH_SZ - 1) -#define __uidhashfn(uid) (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK) -#define uidhashentry(uid) (uidhash_table + __uidhashfn((__kuid_val(uid)))) static struct kmem_cache *uid_cachep; -struct hlist_head uidhash_table[UIDHASH_SZ]; +static DEFINE_HASHTABLE(uidhash_table, UIDHASH_BITS) /* * The uidhash_lock is mostly taken from process context, but it is@@ -84,22 +81,22 @@ struct user_struct root_user = { /* * These routines must be called with the uidhash spinlock held! */ -static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent) +static void uid_hash_insert(struct user_struct *up) { - hlist_add_head(&up->uidhash_node, hashent); + hash_add(uidhash_table, &up->uidhash_node, __kuid_val(up->uid)); } static void uid_hash_remove(struct user_struct *up) { - hlist_del_init(&up->uidhash_node); + hash_del(&up->uidhash_node); } -static struct user_struct *uid_hash_find(kuid_t uid, struct hlist_head *hashent) +static struct user_struct *uid_hash_find(kuid_t uid) { struct user_struct *user; struct hlist_node *h; - hlist_for_each_entry(user, h, hashent, uidhash_node) { + hash_for_each_possible(uidhash_table, user, h, uidhash_node, __kuid_val(uid)) { if (uid_eq(user->uid, uid)) { atomic_inc(&user->__count); return user;@@ -135,7 +132,7 @@ struct user_struct *find_user(kuid_t uid) unsigned long flags; spin_lock_irqsave(&uidhash_lock, flags); - ret = uid_hash_find(uid, uidhashentry(uid)); + ret = uid_hash_find(uid); spin_unlock_irqrestore(&uidhash_lock, flags); return ret; }@@ -156,11 +153,10 @@ void free_uid(struct user_struct *up) struct user_struct *alloc_uid(kuid_t uid) { - struct hlist_head *hashent = uidhashentry(uid); struct user_struct *up, *new; spin_lock_irq(&uidhash_lock); - up = uid_hash_find(uid, hashent); + up = uid_hash_find(uid); spin_unlock_irq(&uidhash_lock); if (!up) {@@ -176,13 +172,13 @@ struct user_struct *alloc_uid(kuid_t uid) * on adding the same user already.. */ spin_lock_irq(&uidhash_lock); - up = uid_hash_find(uid, hashent); + up = uid_hash_find(uid); if (up) { key_put(new->uid_keyring); key_put(new->session_keyring); kmem_cache_free(uid_cachep, new); } else { - uid_hash_insert(new, hashent); + uid_hash_insert(new); up = new; } spin_unlock_irq(&uidhash_lock);@@ -196,17 +192,14 @@ out_unlock: static int __init uid_cache_init(void) { - int n; - uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct), 0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL); - for(n = 0; n < UIDHASH_SZ; ++n) - INIT_HLIST_HEAD(uidhash_table + n); + hash_init(uidhash_table); /* Insert the root user immediately (init already runs as root) */ spin_lock_irq(&uidhash_lock); - uid_hash_insert(&root_user, uidhashentry(GLOBAL_ROOT_UID)); + uid_hash_insert(&root_user); spin_unlock_irq(&uidhash_lock); return 0;
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