Re: [PATCH 22/23] SUNRPC: simplify auth_unix.

[Chuck Lever]

> On Nov 7, 2018, at 8:41 PM, NeilBrown <neilb@xxxxxxxx> wrote:
> 
> On Wed, Nov 07 2018, Chuck Lever wrote:
> 
>> Hi Neil-
>> 
>> 
>>> On Nov 6, 2018, at 11:12 PM, NeilBrown <neilb@xxxxxxxx> wrote:
>>> 
>>> 1/ discard 'struct unx_cred'.  We don't need any data that
>>>  is not already in 'struct rpc_cred'.
>>> 2/ Don't keep these creds in a hash table.  When a credential
>>>  is needed, simply allocate it.  When not needed, discard it.
>>>  This can easily be faster than performing a lookup on
>>>  a shared hash table.
> 
> Thanks for the review Chuck!
> 
>> 
>> What's the basis for this claim? A memory allocation disables and
>> enables IRQs. That definitely hits a resource that is globally
>> shared.
> 
> My basis is not rock solid, but I was convinced :-)
> 
> kmem_cache_alloc() does disable local irqs when slab.c is used.
> slub.c doesn't disable them in the fast path which I *think* should be
> reasonably common.
> slob always takes a spinlock as well as disabling interrupts.
> 
> I think slob is only recommended for tiny machines, and slub is
> generally preferred, so I think that when performance matters, it will
> still be delivered.
> 
> It isn't clear to me why you consider a local irq to be "globally
> shared" - assuming that is what you mean.

Globally-shared in this case can be construed somewhat narrowly.
If we assume slub, kmem_cache_alloc() touches resources that are
used by all tasks running on that CPU, at least in the slow path.


> Disabling local interrupts is not without cost, but I don't think the
> cost increases with the number of CPUs, while the cost of accessing
> shared memory (even without a spinlock) does.

Point taken, but having a single mempool for all RPC transports
and users is also going to be a shared resource that can
bottleneck.

I guess that's an argument in favor of building creds on demand
rather than keeping them in a hash table, isn't it. :-)


>> In addition, the comment near unx_marshal suggests we should
>> cache the marshaled on-the-wire version of the credential instead
>> of building it in the RPC Call buffer every time. That would
>> require keeping the creds around.
> 
> That comment has been there since 2.1.32 and has not be acted on.  There
> seems little reason to expect that to change.  Caching doesn't seem to
> have been found to be necessary in practice.
> 
>> 
>> Have you measured a significant difference in throughput with
>> this patch? Have you considered improving the lookup speed of
>> the hash table by making the buckets into rb-trees, for example?
> 
> No, I haven't measured.
> I might have briefly considered changing to an rb-tree, but as the
> current hashtable doesn't actually contain anything of value, I would
> have quickly discarded the idea.
> 
> If I wanted to further improve performance, I would look at ways to
> bypass the "lookup_cred" step completely.
> unx_marshal only needs the generic "struct cred", so there should be no
> need to have a 'struct rpc_cred' at all.
> 
> If this series is accepted, I'll (hopefully) look into seeing how
> practical that is.
> 
> Thanks again,
> NeilBrown
> 
> 
>> 
>> 
>>>  As the lookup can happen during write-out, use a mempool
>>>  to ensure forward progress.
>>>  This means that we cannot compare two credentials for
>>>  equality by comparing the pointers, but we never do that anyway.
>>> 
>>> Signed-off-by: NeilBrown <neilb@xxxxxxxx>
>>> ---
>>> net/sunrpc/auth.c      |    1 
>>> net/sunrpc/auth_unix.c |  101 +++++++++++++++---------------------------------
>>> 2 files changed, 32 insertions(+), 70 deletions(-)
>>> 
>>> diff --git a/net/sunrpc/auth.c b/net/sunrpc/auth.c
>>> index 867ea9834bde..a07a7c59d3a4 100644
>>> --- a/net/sunrpc/auth.c
>>> +++ b/net/sunrpc/auth.c
>>> @@ -651,6 +651,7 @@ rpcauth_init_cred(struct rpc_cred *cred, const struct auth_cred *acred,
>>> 	INIT_LIST_HEAD(&cred->cr_lru);
>>> 	refcount_set(&cred->cr_count, 1);
>>> 	cred->cr_auth = auth;
>>> +	cred->cr_flags = 0;
>>> 	cred->cr_ops = ops;
>>> 	cred->cr_expire = jiffies;
>>> 	cred->cr_cred = get_cred(acred->cred);
>>> diff --git a/net/sunrpc/auth_unix.c b/net/sunrpc/auth_unix.c
>>> index bff113a411e0..387f6b3ffbea 100644
>>> --- a/net/sunrpc/auth_unix.c
>>> +++ b/net/sunrpc/auth_unix.c
>>> @@ -11,16 +11,11 @@
>>> #include <linux/types.h>
>>> #include <linux/sched.h>
>>> #include <linux/module.h>
>>> +#include <linux/mempool.h>
>>> #include <linux/sunrpc/clnt.h>
>>> #include <linux/sunrpc/auth.h>
>>> #include <linux/user_namespace.h>
>>> 
>>> -struct unx_cred {
>>> -	struct rpc_cred		uc_base;
>>> -	kgid_t			uc_gid;
>>> -	kgid_t			uc_gids[UNX_NGROUPS];
>>> -};
>>> -#define uc_uid			uc_base.cr_uid
>>> 
>>> #if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
>>> # define RPCDBG_FACILITY	RPCDBG_AUTH
>>> @@ -28,6 +23,7 @@ struct unx_cred {
>>> 
>>> static struct rpc_auth		unix_auth;
>>> static const struct rpc_credops	unix_credops;
>>> +static mempool_t		*unix_pool;
>>> 
>>> static struct rpc_auth *
>>> unx_create(const struct rpc_auth_create_args *args, struct rpc_clnt *clnt)
>>> @@ -42,15 +38,6 @@ static void
>>> unx_destroy(struct rpc_auth *auth)
>>> {
>>> 	dprintk("RPC:       destroying UNIX authenticator %p\n", auth);
>>> -	rpcauth_clear_credcache(auth->au_credcache);
>>> -}
>>> -
>>> -static int
>>> -unx_hash_cred(struct auth_cred *acred, unsigned int hashbits)
>>> -{
>>> -	return hash_64(from_kgid(&init_user_ns, acred->cred->fsgid) |
>>> -		((u64)from_kuid(&init_user_ns, acred->cred->fsuid) <<
>>> -			(sizeof(gid_t) * 8)), hashbits);
>>> }
>>> 
>>> /*
>>> @@ -59,53 +46,24 @@ unx_hash_cred(struct auth_cred *acred, unsigned int hashbits)
>>> static struct rpc_cred *
>>> unx_lookup_cred(struct rpc_auth *auth, struct auth_cred *acred, int flags)
>>> {
>>> -	return rpcauth_lookup_credcache(auth, acred, flags, GFP_NOFS);
>>> -}
>>> -
>>> -static struct rpc_cred *
>>> -unx_create_cred(struct rpc_auth *auth, struct auth_cred *acred, int flags, gfp_t gfp)
>>> -{
>>> -	struct unx_cred	*cred;
>>> -	unsigned int groups = 0;
>>> -	unsigned int i;
>>> +	struct rpc_cred *ret = mempool_alloc(unix_pool, GFP_NOFS);
>>> 
>>> 	dprintk("RPC:       allocating UNIX cred for uid %d gid %d\n",
>>> 			from_kuid(&init_user_ns, acred->cred->fsuid),
>>> 			from_kgid(&init_user_ns, acred->cred->fsgid));
>>> 
>>> -	if (!(cred = kmalloc(sizeof(*cred), gfp)))
>>> -		return ERR_PTR(-ENOMEM);
>>> -
>>> -	rpcauth_init_cred(&cred->uc_base, acred, auth, &unix_credops);
>>> -	cred->uc_base.cr_flags = 1UL << RPCAUTH_CRED_UPTODATE;
>>> -
>>> -	if (acred->cred && acred->cred->group_info != NULL)
>>> -		groups = acred->cred->group_info->ngroups;
>>> -	if (groups > UNX_NGROUPS)
>>> -		groups = UNX_NGROUPS;
>>> -
>>> -	cred->uc_gid = acred->cred->fsgid;
>>> -	for (i = 0; i < groups; i++)
>>> -		cred->uc_gids[i] = acred->cred->group_info->gid[i];
>>> -	if (i < UNX_NGROUPS)
>>> -		cred->uc_gids[i] = INVALID_GID;
>>> -
>>> -	return &cred->uc_base;
>>> -}
>>> -
>>> -static void
>>> -unx_free_cred(struct unx_cred *unx_cred)
>>> -{
>>> -	dprintk("RPC:       unx_free_cred %p\n", unx_cred);
>>> -	put_cred(unx_cred->uc_base.cr_cred);
>>> -	kfree(unx_cred);
>>> +	rpcauth_init_cred(ret, acred, auth, &unix_credops);
>>> +	ret->cr_flags = 1UL << RPCAUTH_CRED_UPTODATE;
>>> +	return ret;
>>> }
>>> 
>>> static void
>>> unx_free_cred_callback(struct rcu_head *head)
>>> {
>>> -	struct unx_cred *unx_cred = container_of(head, struct unx_cred, uc_base.cr_rcu);
>>> -	unx_free_cred(unx_cred);
>>> +	struct rpc_cred *rpc_cred = container_of(head, struct rpc_cred, cr_rcu);
>>> +	dprintk("RPC:       unx_free_cred %p\n", rpc_cred);
>>> +	put_cred(rpc_cred->cr_cred);
>>> +	mempool_free(rpc_cred, unix_pool);
>>> }
>>> 
>>> static void
>>> @@ -115,30 +73,32 @@ unx_destroy_cred(struct rpc_cred *cred)
>>> }
>>> 
>>> /*
>>> - * Match credentials against current process creds.
>>> - * The root_override argument takes care of cases where the caller may
>>> - * request root creds (e.g. for NFS swapping).
>>> + * Match credentials against current the auth_cred.
>>> */
>>> static int
>>> -unx_match(struct auth_cred *acred, struct rpc_cred *rcred, int flags)
>>> +unx_match(struct auth_cred *acred, struct rpc_cred *cred, int flags)
>>> {
>>> -	struct unx_cred	*cred = container_of(rcred, struct unx_cred, uc_base);
>>> 	unsigned int groups = 0;
>>> 	unsigned int i;
>>> 
>>> +	if (cred->cr_cred == acred->cred)
>>> +		return 1;
>>> 
>>> -	if (!uid_eq(cred->uc_uid, acred->cred->fsuid) || !gid_eq(cred->uc_gid, acred->cred->fsgid))
>>> +	if (!uid_eq(cred->cr_cred->fsuid, acred->cred->fsuid) || !gid_eq(cred->cr_cred->fsgid, acred->cred->fsgid))
>>> 		return 0;
>>> 
>>> 	if (acred->cred && acred->cred->group_info != NULL)
>>> 		groups = acred->cred->group_info->ngroups;
>>> 	if (groups > UNX_NGROUPS)
>>> 		groups = UNX_NGROUPS;
>>> +	if (cred->cr_cred->group_info == NULL)
>>> +		return groups == 0;
>>> +	if (groups != cred->cr_cred->group_info->ngroups)
>>> +		return 0;
>>> +
>>> 	for (i = 0; i < groups ; i++)
>>> -		if (!gid_eq(cred->uc_gids[i], acred->cred->group_info->gid[i]))
>>> +		if (!gid_eq(cred->cr_cred->group_info->gid[i], acred->cred->group_info->gid[i]))
>>> 			return 0;
>>> -	if (groups < UNX_NGROUPS && gid_valid(cred->uc_gids[groups]))
>>> -		return 0;
>>> 	return 1;
>>> }
>>> 
>>> @@ -150,9 +110,10 @@ static __be32 *
>>> unx_marshal(struct rpc_task *task, __be32 *p)
>>> {
>>> 	struct rpc_clnt	*clnt = task->tk_client;
>>> -	struct unx_cred	*cred = container_of(task->tk_rqstp->rq_cred, struct unx_cred, uc_base);
>>> +	struct rpc_cred	*cred = task->tk_rqstp->rq_cred;
>>> 	__be32		*base, *hold;
>>> 	int		i;
>>> +	struct group_info *gi = cred->cr_cred->group_info;
>>> 
>>> 	*p++ = htonl(RPC_AUTH_UNIX);
>>> 	base = p++;
>>> @@ -163,11 +124,12 @@ unx_marshal(struct rpc_task *task, __be32 *p)
>>> 	 */
>>> 	p = xdr_encode_array(p, clnt->cl_nodename, clnt->cl_nodelen);
>>> 
>>> -	*p++ = htonl((u32) from_kuid(&init_user_ns, cred->uc_uid));
>>> -	*p++ = htonl((u32) from_kgid(&init_user_ns, cred->uc_gid));
>>> +	*p++ = htonl((u32) from_kuid(&init_user_ns, cred->cr_cred->fsuid));
>>> +	*p++ = htonl((u32) from_kgid(&init_user_ns, cred->cr_cred->fsgid));
>>> 	hold = p++;
>>> -	for (i = 0; i < UNX_NGROUPS && gid_valid(cred->uc_gids[i]); i++)
>>> -		*p++ = htonl((u32) from_kgid(&init_user_ns, cred->uc_gids[i]));
>>> +	if (gi)
>>> +		for (i = 0; i < UNX_NGROUPS && i < gi->ngroups; i++)
>>> +			*p++ = htonl((u32) from_kgid(&init_user_ns, gi->gid[i]));
>>> 	*hold = htonl(p - hold - 1);		/* gid array length */
>>> 	*base = htonl((p - base - 1) << 2);	/* cred length */
>>> 
>>> @@ -214,12 +176,13 @@ unx_validate(struct rpc_task *task, __be32 *p)
>>> 
>>> int __init rpc_init_authunix(void)
>>> {
>>> -	return rpcauth_init_credcache(&unix_auth);
>>> +	unix_pool = mempool_create_kmalloc_pool(16, sizeof(struct rpc_cred));
>>> +	return unix_pool ? 0 : -ENOMEM;
>>> }
>>> 
>>> void rpc_destroy_authunix(void)
>>> {
>>> -	rpcauth_destroy_credcache(&unix_auth);
>>> +	mempool_destroy(unix_pool);
>>> }
>>> 
>>> const struct rpc_authops authunix_ops = {
>>> @@ -228,9 +191,7 @@ const struct rpc_authops authunix_ops = {
>>> 	.au_name	= "UNIX",
>>> 	.create		= unx_create,
>>> 	.destroy	= unx_destroy,
>>> -	.hash_cred	= unx_hash_cred,
>>> 	.lookup_cred	= unx_lookup_cred,
>>> -	.crcreate	= unx_create_cred,
>>> };
>>> 
>>> static
>>> 
>>> 
>> 
>> --
>> Chuck Lever

--
Chuck Lever