> On Sep 3, 2024, at 11:29 AM, Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
>
> On Tue, Sep 03, 2024 at 11:19:45AM -0400, Jeff Layton wrote:
>> On Tue, 2024-09-03 at 11:00 -0400, Mike Snitzer wrote:
>>> On Tue, Sep 03, 2024 at 10:40:28AM -0400, Jeff Layton wrote:
>>>> On Tue, 2024-09-03 at 10:34 -0400, Chuck Lever wrote:
>>>>> On Sat, Aug 31, 2024 at 06:37:36PM -0400, Mike Snitzer wrote:
>>>>>> From: Weston Andros Adamson <dros@xxxxxxxxxxxxxxx>
>>>>>>
>>>>>> Add server support for bypassing NFS for localhost reads, writes, and
>>>>>> commits. This is only useful when both the client and server are
>>>>>> running on the same host.
>>>>>>
>>>>>> If nfsd_open_local_fh() fails then the NFS client will both retry and
>>>>>> fallback to normal network-based read, write and commit operations if
>>>>>> localio is no longer supported.
>>>>>>
>>>>>> Care is taken to ensure the same NFS security mechanisms are used
>>>>>> (authentication, etc) regardless of whether localio or regular NFS
>>>>>> access is used. The auth_domain established as part of the traditional
>>>>>> NFS client access to the NFS server is also used for localio. Store
>>>>>> auth_domain for localio in nfsd_uuid_t and transfer it to the client
>>>>>> if it is local to the server.
>>>>>>
>>>>>> Relative to containers, localio gives the client access to the network
>>>>>> namespace the server has. This is required to allow the client to
>>>>>> access the server's per-namespace nfsd_net struct.
>>>>>>
>>>>>> This commit also introduces the use of NFSD's percpu_ref to interlock
>>>>>> nfsd_destroy_serv and nfsd_open_local_fh, to ensure nn->nfsd_serv is
>>>>>> not destroyed while in use by nfsd_open_local_fh and other LOCALIO
>>>>>> client code.
>>>>>>
>>>>>> CONFIG_NFS_LOCALIO enables NFS server support for LOCALIO.
>>>>>>
>>>>>> Signed-off-by: Weston Andros Adamson <dros@xxxxxxxxxxxxxxx>
>>>>>> Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
>>>>>> Co-developed-by: Mike Snitzer <snitzer@xxxxxxxxxx>
>>>>>> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
>>>>>> Co-developed-by: NeilBrown <neilb@xxxxxxx>
>>>>>> Signed-off-by: NeilBrown <neilb@xxxxxxx>
>>>>>>
>>>>>> Not-Acked-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
>>>>>> Not-Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
>>>>>> ---
>>>>>> fs/nfsd/Makefile | 1 +
>>>>>> fs/nfsd/filecache.c | 2 +-
>>>>>> fs/nfsd/localio.c | 112 +++++++++++++++++++++++++++++++++++++
>>>>>> fs/nfsd/netns.h | 4 ++
>>>>>> fs/nfsd/nfsctl.c | 25 ++++++++-
>>>>>> fs/nfsd/trace.h | 3 +-
>>>>>> fs/nfsd/vfs.h | 2 +
>>>>>> include/linux/nfslocalio.h | 8 +++
>>>>>> 8 files changed, 154 insertions(+), 3 deletions(-)
>>>>>> create mode 100644 fs/nfsd/localio.c
>>>>>>
>>>>>> diff --git a/fs/nfsd/Makefile b/fs/nfsd/Makefile
>>>>>> index b8736a82e57c..18cbd3fa7691 100644
>>>>>> --- a/fs/nfsd/Makefile
>>>>>> +++ b/fs/nfsd/Makefile
>>>>>> @@ -23,3 +23,4 @@ nfsd-$(CONFIG_NFSD_PNFS) += nfs4layouts.o
>>>>>> nfsd-$(CONFIG_NFSD_BLOCKLAYOUT) += blocklayout.o blocklayoutxdr.o
>>>>>> nfsd-$(CONFIG_NFSD_SCSILAYOUT) += blocklayout.o blocklayoutxdr.o
>>>>>> nfsd-$(CONFIG_NFSD_FLEXFILELAYOUT) += flexfilelayout.o flexfilelayoutxdr.o
>>>>>> +nfsd-$(CONFIG_NFS_LOCALIO) += localio.o
>>>>>> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
>>>>>> index 89ff380ec31e..348c1b97092e 100644
>>>>>> --- a/fs/nfsd/filecache.c
>>>>>> +++ b/fs/nfsd/filecache.c
>>>>>> @@ -52,7 +52,7 @@
>>>>>> #define NFSD_FILE_CACHE_UP (0)
>>>>>>
>>>>>> /* We only care about NFSD_MAY_READ/WRITE for this cache */
>>>>>> -#define NFSD_FILE_MAY_MASK (NFSD_MAY_READ|NFSD_MAY_WRITE)
>>>>>> +#define NFSD_FILE_MAY_MASK (NFSD_MAY_READ|NFSD_MAY_WRITE|NFSD_MAY_LOCALIO)
>>>>>>
>>>>>> static DEFINE_PER_CPU(unsigned long, nfsd_file_cache_hits);
>>>>>> static DEFINE_PER_CPU(unsigned long, nfsd_file_acquisitions);
>>>>>> diff --git a/fs/nfsd/localio.c b/fs/nfsd/localio.c
>>>>>> new file mode 100644
>>>>>> index 000000000000..75df709c6903
>>>>>> --- /dev/null
>>>>>> +++ b/fs/nfsd/localio.c
>>>>>> @@ -0,0 +1,112 @@
>>>>>> +// SPDX-License-Identifier: GPL-2.0-only
>>>>>> +/*
>>>>>> + * NFS server support for local clients to bypass network stack
>>>>>> + *
>>>>>> + * Copyright (C) 2014 Weston Andros Adamson <dros@xxxxxxxxxxxxxxx>
>>>>>> + * Copyright (C) 2019 Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
>>>>>> + * Copyright (C) 2024 Mike Snitzer <snitzer@xxxxxxxxxxxxxxx>
>>>>>> + * Copyright (C) 2024 NeilBrown <neilb@xxxxxxx>
>>>>>> + */
>>>>>> +
>>>>>> +#include <linux/exportfs.h>
>>>>>> +#include <linux/sunrpc/svcauth.h>
>>>>>> +#include <linux/sunrpc/clnt.h>
>>>>>> +#include <linux/nfs.h>
>>>>>> +#include <linux/nfs_common.h>
>>>>>> +#include <linux/nfslocalio.h>
>>>>>> +#include <linux/string.h>
>>>>>> +
>>>>>> +#include "nfsd.h"
>>>>>> +#include "vfs.h"
>>>>>> +#include "netns.h"
>>>>>> +#include "filecache.h"
>>>>>> +
>>>>>> +static const struct nfsd_localio_operations nfsd_localio_ops = {
>>>>>> + .nfsd_open_local_fh = nfsd_open_local_fh,
>>>>>> + .nfsd_file_put_local = nfsd_file_put_local,
>>>>>> + .nfsd_file_file = nfsd_file_file,
>>>>>> +};
>>>>>> +
>>>>>> +void nfsd_localio_ops_init(void)
>>>>>> +{
>>>>>> + memcpy(&nfs_to, &nfsd_localio_ops, sizeof(nfsd_localio_ops));
>>>>>> +}
>>>>>
>>>>> Same comment as Neil: this should surface a pointer to the
>>>>> localio_ops struct. Copying the whole set of function pointers is
>>>>> generally unnecessary.
>>>>>
>>>>>
>>>>>> +
>>>>>> +/**
>>>>>> + * nfsd_open_local_fh - lookup a local filehandle @nfs_fh and map to nfsd_file
>>>>>> + *
>>>>>> + * @uuid: nfs_uuid_t which provides the 'struct net' to get the proper nfsd_net
>>>>>> + * and the 'struct auth_domain' required for LOCALIO access
>>>>>> + * @rpc_clnt: rpc_clnt that the client established, used for sockaddr and cred
>>>>>> + * @cred: cred that the client established
>>>>>> + * @nfs_fh: filehandle to lookup
>>>>>> + * @fmode: fmode_t to use for open
>>>>>> + *
>>>>>> + * This function maps a local fh to a path on a local filesystem.
>>>>>> + * This is useful when the nfs client has the local server mounted - it can
>>>>>> + * avoid all the NFS overhead with reads, writes and commits.
>>>>>> + *
>>>>>> + * On successful return, returned nfsd_file will have its nf_net member
>>>>>> + * set. Caller (NFS client) is responsible for calling nfsd_serv_put and
>>>>>> + * nfsd_file_put (via nfs_to.nfsd_file_put_local).
>>>>>> + */
>>>>>> +struct nfsd_file *
>>>>>> +nfsd_open_local_fh(nfs_uuid_t *uuid,
>>>>>> + struct rpc_clnt *rpc_clnt, const struct cred *cred,
>>>>>> + const struct nfs_fh *nfs_fh, const fmode_t fmode)
>>>>>> + __must_hold(rcu)
>>>>>> +{
>>>>>> + int mayflags = NFSD_MAY_LOCALIO;
>>>>>> + struct nfsd_net *nn = NULL;
>>>>>> + struct net *net;
>>>>>> + struct svc_cred rq_cred;
>>>>>> + struct svc_fh fh;
>>>>>> + struct nfsd_file *localio;
>>>>>> + __be32 beres;
>>>>>> +
>>>>>> + if (nfs_fh->size > NFS4_FHSIZE)
>>>>>> + return ERR_PTR(-EINVAL);
>>>>>> +
>>>>>> + /*
>>>>>> + * Not running in nfsd context, so must safely get reference on nfsd_serv.
>>>>>> + * But the server may already be shutting down, if so disallow new localio.
>>>>>> + * uuid->net is NOT a counted reference, but caller's rcu_read_lock() ensures
>>>>>> + * that if uuid->net is not NULL, then calling nfsd_serv_try_get() is safe
>>>>>> + * and if it succeeds we will have an implied reference to the net.
>>>>>> + */
>>>>>> + net = rcu_dereference(uuid->net);
>>>>>> + if (net)
>>>>>> + nn = net_generic(net, nfsd_net_id);
>>>>>> + if (unlikely(!nn || !nfsd_serv_try_get(nn)))
>>>>>> + return ERR_PTR(-ENXIO);
>>>>>> +
>>>>>> + /* Drop the rcu lock for nfsd_file_acquire_local() */
>>>>>> + rcu_read_unlock();
>>>>>
>>>>> I'm struggling with the locking logistics. Caller takes the RCU read
>>>>> lock, this function drops the lock, then takes it again. So:
>>>>>
>>>>> - A caller might rely on the lock being held continuously, but
>>>>> - The API contract documented above doesn't indicate that this
>>>>> function drops that lock
>>>>> - The __must_hold(rcu) annotation doesn't indicate that this
>>>>> function drops that lock, IIUC
>>>>>
>>>>> Dropping and retaking the lock in here is an anti-pattern that
>>>>> should be avoided. I suggest we are better off in the long run if
>>>>> the caller does not need to take the RCU read lock, but instead,
>>>>> nfsd_open_local_fh takes it right here just for the rcu_dereference.
>>>
>>> I thought so too when I first saw how Neil approached fixing this to
>>> be safe. It was only after putting further time to it (and having the
>>> benefit of being so close to all this) that I realized the nuance at
>>> play (please see my reply to Jeff below for the nuance I'm speaking
>>> of).
>>>
>>>>>
>>>>> OTOH, Why drop the lock before calling nfsd_file_acquire_local()?
>>>>> The RCU read lock can safely be taken more than once in succession.
>>>>>
>>>>> Let's rethink the locking strategy.
>>>>>
>>>
>>> Yes, _that_ is a very valid point. I did wonder the same: it seems
>>> perfectly fine to simply retain the RCU throughout the entirety of
>>> nfsd_open_local_fh().
>>>
>>
>> Nope. nfsd_file_do_acquire can allocate, so you can't hold the
>> rcu_read_lock over the whole thing.
>
> Ah, yeap.. sorry, I knew that ;)
>
>>>> Agreed. The only caller does this:
>>>>
>>>> rcu_read_lock();
>>>> if (!rcu_access_pointer(uuid->net)) {
>>>> rcu_read_unlock();
>>>> return ERR_PTR(-ENXIO);
>>>> }
>>>> localio = nfs_to.nfsd_open_local_fh(uuid, rpc_clnt, cred,
>>>> nfs_fh, fmode);
>>>> rcu_read_unlock();
>>>>
>>>> Maybe just move the check for uuid->net down into nfsd_open_local_fh,
>>>> and it can acquire the rcu_read_lock for itself?
>>>
>>> No, sorry we cannot. The call to nfs_to.nfsd_open_local_fh (which is
>>> a symbol provided by nfsd) is only safe if the RCU protected pre-check
>>> shows the uuid->net valid.
>>
>> Ouch, ok.
>
> I had to double check but I did add a comment that speaks directly to
> this "nuance" above the code you quoted:
>
> /*
> * uuid->net must not be NULL, otherwise NFS may not have ref
> * on NFSD and therefore cannot safely make 'nfs_to' calls.
> */
>
> So yeah, this code needs to stay like this. The __must_hold(rcu) just
> ensures the RCU is held on entry and exit.. the bouncing of RCU
> (dropping and retaking) isn't of immediate concern is it? While I
> agree it isn't ideal, it is what it is given:
> 1) NFS caller of NFSD symbol is only safe if it has RCU amd verified
> uuid->net valid
> 2) nfsd_file_do_acquire() can allocate.
OK, understood, but the annotation is still wrong. The lock
is dropped here so I think you need __releases and __acquires
in that case. However...
Let's wait for Neil's comments, but I think this needs to be
properly addressed before merging. The comments are not going
to be enough IMO.
--
Chuck Lever
