linux-kernel - Re: [Devel] [PATCH 2/6] nfsd: swap fs root in NFSd kthreads

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Message-ID: <50C74877.4070200@parallels.com>
Date:	Tue, 11 Dec 2012 18:51:35 +0400
From:	Stanislav Kinsbursky <skinsbursky@...allels.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
CC:	<linux-nfs@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<devel@...nvz.org>
Subject: Re: [Devel] [PATCH 2/6] nfsd: swap fs root in NFSd kthreads

11.12.2012 18:12, Stanislav Kinsbursky пишет:
> 11.12.2012 18:00, Stanislav Kinsbursky пишет:
>> 11.12.2012 00:28, J. Bruce Fields пишет:
>>> On Thu, Dec 06, 2012 at 06:34:47PM +0300, Stanislav Kinsbursky wrote:
>>>> NFSd does lookup. Lookup is done starting from current->fs->root.
>>>> NFSd is a kthread, cloned by kthreadd, and thus have global (but luckely
>>>> unshared) root.
>>>> So we have to swap root to those, which process, started NFSd, has. Because
>>>> that process can be in a container with it's own root.
>>>
>>> This doesn't sound right to me.
>>>
>>> Which lookups exactly do you see being done relative to
>>> current->fs->root ?
>>>
>>
>> Ok, you are right. I was mistaken here.
>> This is not a exactly lookup, but d_path() problem in svc_export_request().
>> I.e. without root swapping, d_path() will give not local export path (like "/export")
>> but something like this "/root/containers_root/export".
>>
>
> We, actually, can do it less in less aggressive way.
> I.e. instead root swap and current svc_export_request() implementation:
>
> void svc_export_request(...)
> {
>      <snip>
>          pth = d_path(&exp->ex_path, *bpp, *blen);
>      <snip>
> }
>
> we can do something like this:
>
> void svc_export_request(...)
> {
>      struct nfsd_net *nn = ...
>      <snip>
>      spin_lock(&dcache_lock);
>          pth = __d_path(&exp->ex_path, &nn->root, *bpp, *blen);
>      spin_unlock(&dcache_lock);
>      <snip>
> }
>

No, this won't work. Sorry for noise.

>>> --b.
>>>
>>>>
>>>> Signed-off-by: Stanislav Kinsbursky <skinsbursky@...allels.com>
>>>> ---
>>>>   fs/nfsd/netns.h  |    1 +
>>>>   fs/nfsd/nfssvc.c |   33 ++++++++++++++++++++++++++++++++-
>>>>   2 files changed, 33 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
>>>> index abfc97c..5c423c6 100644
>>>> --- a/fs/nfsd/netns.h
>>>> +++ b/fs/nfsd/netns.h
>>>> @@ -101,6 +101,7 @@ struct nfsd_net {
>>>>       struct timeval nfssvc_boot;
>>>>
>>>>       struct svc_serv *nfsd_serv;
>>>> +    struct path    root;
>>>>   };
>>>>
>>>>   extern int nfsd_net_id;
>>>> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
>>>> index cee62ab..177bb60 100644
>>>> --- a/fs/nfsd/nfssvc.c
>>>> +++ b/fs/nfsd/nfssvc.c
>>>> @@ -392,6 +392,7 @@ int nfsd_create_serv(struct net *net)
>>>>
>>>>       set_max_drc();
>>>>       do_gettimeofday(&nn->nfssvc_boot);        /* record boot time */
>>>> +    get_fs_root(current->fs, &nn->root);
>>>>       return 0;
>>>>   }
>>>>
>>>> @@ -426,8 +427,10 @@ void nfsd_destroy(struct net *net)
>>>>       if (destroy)
>>>>           svc_shutdown_net(nn->nfsd_serv, net);
>>>>       svc_destroy(nn->nfsd_serv);
>>>> -    if (destroy)
>>>> +    if (destroy) {
>>>> +        path_put(&nn->root);
>>>>           nn->nfsd_serv = NULL;
>>>> +    }
>>>>   }
>>>>
>>>>   int nfsd_set_nrthreads(int n, int *nthreads, struct net *net)
>>>> @@ -533,6 +536,25 @@ out:
>>>>       return error;
>>>>   }
>>>>
>>>> +/*
>>>> + * This function is actually slightly modified set_fs_root()<F9>
>>>> + */
>>>> +static void nfsd_swap_root(struct net *net)
>>>> +{
>>>> +    struct nfsd_net *nn = net_generic(net, nfsd_net_id);
>>>> +    struct fs_struct *fs = current->fs;
>>>> +    struct path old_root;
>>>> +
>>>> +    path_get(&nn->root);
>>>> +    spin_lock(&fs->lock);
>>>> +    write_seqcount_begin(&fs->seq);
>>>> +    old_root = fs->root;
>>>> +    fs->root = nn->root;
>>>> +    write_seqcount_end(&fs->seq);
>>>> +    spin_unlock(&fs->lock);
>>>> +    if (old_root.dentry)
>>>> +        path_put(&old_root);
>>>> +}
>>>>
>>>>   /*
>>>>    * This is the NFS server kernel thread
>>>> @@ -559,6 +581,15 @@ nfsd(void *vrqstp)
>>>>       current->fs->umask = 0;
>>>>
>>>>       /*
>>>> +     * We have to swap NFSd kthread's fs->root.
>>>> +     * Why so? Because NFSd can be started in container, which has it's own
>>>> +     * root.
>>>> +     * And so what? NFSd lookup files, and lookup start from
>>>> +     * current->fs->root.
>>>> +     */
>>>> +    nfsd_swap_root(net);
>>>> +
>>>> +    /*
>>>>        * thread is spawned with all signals set to SIG_IGN, re-enable
>>>>        * the ones that will bring down the thread
>>>>        */
>>>>
>>
>>
>
>


-- 
Best regards,
Stanislav Kinsbursky
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