Re: [for-6.13 PATCH 10/19] nfs_common: move localio_lock to new lock member of nfs_uuid_t

NeilBrown
2024-11-11T20:35:12Z

On Tue, 12 Nov 2024, Mike Snitzer wrote:
> On Mon, Nov 11, 2024 at 12:55:24PM +1100, NeilBrown wrote:
> > On Sat, 09 Nov 2024, Mike Snitzer wrote:
> > > Remove cl_localio_lock from 'struct nfs_client' in favor of adding a
> > > lock to the nfs_uuid_t struct (which is embedded in each nfs_client).
> > >
> > > Push nfs_local_{enable,disable} implementation down to nfs_common.
> > > Those methods now call nfs_localio_{enable,disable}_client.
> > >
> > > This allows implementing nfs_localio_invalidate_clients in terms of
> > > nfs_localio_disable_client.
> > >
> > > Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> > > ---
> > > fs/nfs/client.c | 1 -
> > > fs/nfs/localio.c | 18 ++++++------
> > > fs/nfs_common/nfslocalio.c | 57 ++++++++++++++++++++++++++------------
> > > include/linux/nfs_fs_sb.h | 1 -
> > > include/linux/nfslocalio.h | 8 +++++-
> > > 5 files changed, 55 insertions(+), 30 deletions(-)
> > >
> > > diff --git a/fs/nfs/client.c b/fs/nfs/client.c
> > > index 03ecc7765615..124232054807 100644
> > > --- a/fs/nfs/client.c
> > > +++ b/fs/nfs/client.c
> > > @@ -182,7 +182,6 @@ struct nfs_client *nfs_alloc_client(const struct nfs_client_initdata *cl_init)
> > > seqlock_init(&clp->cl_boot_lock);
> > > ktime_get_real_ts64(&clp->cl_nfssvc_boot);
> > > nfs_uuid_init(&clp->cl_uuid);
> > > - spin_lock_init(&clp->cl_localio_lock);
> > > #endif /* CONFIG_NFS_LOCALIO */
> > >
> > > clp->cl_principal = "*";
> > > diff --git a/fs/nfs/localio.c b/fs/nfs/localio.c
> > > index cab2a8819259..4c75ffc5efa2 100644
> > > --- a/fs/nfs/localio.c
> > > +++ b/fs/nfs/localio.c
> > > @@ -125,10 +125,8 @@ const struct rpc_program nfslocalio_program = {
> > > */
> > > static void nfs_local_enable(struct nfs_client *clp)
> > > {
> > > - spin_lock(&clp->cl_localio_lock);
> > > - set_bit(NFS_CS_LOCAL_IO, &clp->cl_flags);
> > > trace_nfs_local_enable(clp);
> > > - spin_unlock(&clp->cl_localio_lock);
> > > + nfs_localio_enable_client(clp);
> > > }
> >
> > Why do we need this function? The one caller could call
> > nfs_localio_enable_client() directly instead. The tracepoint could be
> > placed in that one caller.
>
> Yeah, I saw that too but felt it useful to differentiate between calls
> that occur during NFS client initialization and those that happen as a
> side-effect of callers from other contexts (in later patch this
> manifests as nfs_localio_disable_client vs nfs_local_disable).
>
> Hence my adding secondary tracepoints for nfs_common (see "[PATCH
> 17/19] nfs_common: add nfs_localio trace events).
>
> But sure, we can just eliminate nfs_local_{enable,disable} and the
> corresponding tracepoints (which will have moved down to nfs_common).

I don't feel strongly about this. If you think these is value in these
wrapper functions then I won't argue. As a general rule I don't like
multiple interfaces that do (much) the same thing as keeping track of
them increases the mental load.

>
> > > /*
> > > @@ -136,12 +134,8 @@ static void nfs_local_enable(struct nfs_client *clp)
> > > */
> > > void nfs_local_disable(struct nfs_client *clp)
> > > {
> > > - spin_lock(&clp->cl_localio_lock);
> > > - if (test_and_clear_bit(NFS_CS_LOCAL_IO, &clp->cl_flags)) {
> > > - trace_nfs_local_disable(clp);
> > > - nfs_localio_disable_client(&clp->cl_uuid);
> > > - }
> > > - spin_unlock(&clp->cl_localio_lock);
> > > + trace_nfs_local_disable(clp);
> > > + nfs_localio_disable_client(clp);
> > > }
> >
> > Ditto. Though there are more callers so the tracepoint solution isn't
> > quite so obvious.
>
> Right... as I just explained: that's why I preserved nfs_local_disable
> (and the tracepoint).
>
>
> > > /*
> > > @@ -183,8 +177,12 @@ static bool nfs_server_uuid_is_local(struct nfs_client *clp)
> > > rpc_shutdown_client(rpcclient_localio);
> > >
> > > /* Server is only local if it initialized required struct members */
> > > - if (status || !clp->cl_uuid.net || !clp->cl_uuid.dom)
> > > + rcu_read_lock();
> > > + if (status || !rcu_access_pointer(clp->cl_uuid.net) || !clp->cl_uuid.dom) {
> > > + rcu_read_unlock();
> > > return false;
> > > + }
> > > + rcu_read_unlock();
> >
> > What value does RCU provide here? I don't think this change is needed.
> > rcu_access_pointer does not require rcu_read_lock().
>
> OK, not sure why I though RCU read-side needed for rcu_access_pointer()...
>
> > > return true;
> > > }
> > > diff --git a/fs/nfs_common/nfslocalio.c b/fs/nfs_common/nfslocalio.c
> > > index 904439e4bb85..cf2f47ea4f8d 100644
> > > --- a/fs/nfs_common/nfslocalio.c
> > > +++ b/fs/nfs_common/nfslocalio.c
> > > @@ -7,6 +7,9 @@
> > > #include <linux/module.h>
> > > #include <linux/list.h>
> > > #include <linux/nfslocalio.h>
> > > +#include <linux/nfs3.h>
> > > +#include <linux/nfs4.h>
> > > +#include <linux/nfs_fs_sb.h>
> >
> > I don't feel good about adding this nfs client knowledge in to nfs_common.
>
> I hear you.. I was "OK with it".
>
> > > #include <net/netns/generic.h>
> > >
> > > MODULE_LICENSE("GPL");
> > > @@ -25,6 +28,7 @@ void nfs_uuid_init(nfs_uuid_t *nfs_uuid)
> > > nfs_uuid->net = NULL;
> > > nfs_uuid->dom = NULL;
> > > INIT_LIST_HEAD(&nfs_uuid->list);
> > > + spin_lock_init(&nfs_uuid->lock);
> > > }
> > > EXPORT_SYMBOL_GPL(nfs_uuid_init);
> > >
> > > @@ -94,12 +98,23 @@ void nfs_uuid_is_local(const uuid_t *uuid, struct list_head *list,
> > > }
> > > EXPORT_SYMBOL_GPL(nfs_uuid_is_local);
> > >
> > > +void nfs_localio_enable_client(struct nfs_client *clp)
> > > +{
> > > + nfs_uuid_t *nfs_uuid = &clp->cl_uuid;
> > > +
> > > + spin_lock(&nfs_uuid->lock);
> > > + set_bit(NFS_CS_LOCAL_IO, &clp->cl_flags);
> > > + spin_unlock(&nfs_uuid->lock);
> > > +}
> > > +EXPORT_SYMBOL_GPL(nfs_localio_enable_client);
> >
> > And I don't feel good about nfs_local accessing nfs_client directly.
> > It only uses it for NFS_CS_LOCAL_IO. Can we ditch that flag and instead
> > so something like testing nfs_uuid.net ??
>
> That'd probably be OK for the equivalent of NFS_CS_LOCAL_IO but the last
> patch in this series ("nfs: probe for LOCALIO when v3 client
> reconnects to server") adds NFS_CS_LOCAL_IO_CAPABLE to provide a hint
> that the client and server successfully established themselves local
> via LOCALIO protocol. This is needed so that NFSv3 (stateless) has a
> hint that reestablishing LOCALIO needed if/when the client loses
> connectivity to the server (because it was shutdown and restarted).

I don't like NFS_CS_LOCAL_IO_CAPABLE.
A use case that I imagine (and a customer does something like this) is an
HA cluster where the NFS server can move from one node to another. All
the node access the filesystem, most over NFS. If a server-migration
happens (e.g. the current server node failed) then the new server node
would suddenly become LOCALIO-capable even though it wasn't at
mount-time. I would like it to be able to detect this and start doing
localio.

So I don't want NFS_CS_LOCAL_IO_CAPABLE. I think tracking when the
network connection is re-established is sufficient.

Thanks,
NeilBrown