linux-kernel - Re: [PATCH v2 10/12] nfsd: use boottime for lease expiry alculation

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Date:   Fri, 13 Dec 2019 17:40:39 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Chuck Lever <chuck.lever@...cle.com>
Cc:     Bruce Fields <bfields@...ldses.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>
Subject: Re: [PATCH v2 10/12] nfsd: use boottime for lease expiry alculation

On Fri, Dec 13, 2019 at 5:26 PM Chuck Lever <chuck.lever@...cle.com> wrote:
> > On Dec 13, 2019, at 9:10 AM, Arnd Bergmann <arnd@...db.de> wrote:

> > diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
> > index 24534db87e86..508d7c6c00b5 100644
> > --- a/fs/nfsd/nfs4callback.c
> > +++ b/fs/nfsd/nfs4callback.c
> > @@ -823,7 +823,12 @@ static const struct rpc_program cb_program = {
> > static int max_cb_time(struct net *net)
> > {
> >       struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> > -     return max(nn->nfsd4_lease/10, (time_t)1) * HZ;
> > +
> > +     /* nfsd4_lease is set to at most one hour */
> > +     if (WARN_ON_ONCE(nn->nfsd4_lease > 3600))
> > +             return 360 * HZ;
>
> Why is the WARN_ON_ONCE added here? Is it really necessary?

This is to ensure the kernel doesn't change to a larger limit that
requires a 64-bit division on a 32-bit architecture.

With the old code, dividing by 10 was always fast as
nn->nfsd4_lease was the size of an integer register. Now it
is 64 bit wide, and I check that truncating it to 32 bit again
is safe.

> (Otherwise these all LGTM).

Thanks for taking a look.

      Arnd