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Subject: Re: [PATCH v2 10/12] nfsd: use boottime for lease expiry alculation
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <CAK8P3a3RXqVqpeTmOrEGtXyeMGZV+5g_QzywGgLnfvi2GMDx=g@mail.gmail.com>
Date:   Fri, 13 Dec 2019 13:23:08 -0500
Cc:     Bruce Fields <bfields@fieldses.org>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        y2038 Mailman List <y2038@lists.linaro.org>
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References: <20191213141046.1770441-1-arnd@arndb.de>
 <20191213141046.1770441-11-arnd@arndb.de>
 <CBC9899C-12BE-466E-8809-EA928AAE1F11@oracle.com>
 <CAK8P3a3RXqVqpeTmOrEGtXyeMGZV+5g_QzywGgLnfvi2GMDx=g@mail.gmail.com>
To:     Arnd Bergmann <arnd@arndb.de>
Sender: linux-nfs-owner@vger.kernel.org
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> On Dec 13, 2019, at 11:40 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> 
> On Fri, Dec 13, 2019 at 5:26 PM Chuck Lever <chuck.lever@oracle.com> wrote:
>>> On Dec 13, 2019, at 9:10 AM, Arnd Bergmann <arnd@arndb.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.

OK. That comment should state this reason rather than just repeating
what the code does. ;-)


>> (Otherwise these all LGTM).
> 
> Thanks for taking a look.
> 
>      Arnd

--
Chuck Lever