Subject: Re: [PATCH 02/16] user_ns: use new hashtable implementation

From: [email-masked] (Eric W. Biederman)
Date: Tue, 14 Aug 2012 16:52:03 -0700

Sasha Levin <[email-masked]> 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

> Signed-off-by: Sasha Levin <[email-masked]>
> ---
>  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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