linux-kernel - Re: [PATCH 05/16] nfs: don't open in ->d

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Date:	Thu, 29 Mar 2012 14:19:08 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
Cc:	"viro@...IV.linux.org.uk" <viro@...iv.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"sfrench@...ba.org" <sfrench@...ba.org>,
	"sage@...dream.net" <sage@...dream.net>,
	"ericvh@...il.com" <ericvh@...il.com>,
	"mszeredi@...e.cz" <mszeredi@...e.cz>
Subject: Re: [PATCH 05/16] nfs: don't open in ->d_revalidate

On Wed, Mar 28, 2012 at 11:45 PM, Myklebust, Trond
<Trond.Myklebust@...app.com> wrote:
> On Wed, 2012-03-28 at 22:24 +0200, Miklos Szeredi wrote:
>> From: Miklos Szeredi <mszeredi@...e.cz>
>>
>> NFSv4 can't do reliable opens in d_revalidate, since it cannot know whether a
>> mount needs to be followed or not.  It does check d_mountpoint() on the dentry,
>> which can result in a weird error if the VFS found that the mount does not in
>> fact need to be followed, e.g.:
>>
>>   # mount --bind /mnt/nfs /mnt/nfs-clone
>>   # echo something > /mnt/nfs/tmp/bar
>>   # echo x > /tmp/file
>>   # mount --bind /tmp/file /mnt/nfs-clone/tmp/bar
>>   # cat  /mnt/nfs/tmp/bar
>>   cat: /mnt/nfs/tmp/bar: Not a directory
>>
>> Which should, by any sane filesystem, result in "something" being printed.
>>
>> So instead do the open in f_op->open() and in the unlikely case that the cached
>> dentry turned out to be invalid, drop the dentry and return ESTALE to let the
>> VFS retry.
>
>
> Just one comment. Would it now make sense for NFSv4 to just skip
> ->d_revalidate() if LOOKUP_OPEN is set, and LOOKUP_EXCL is not set? We
> will in any case be doing a revalidation in nfs4_file_open.

And dentry is positive and regular.  Which is basically what
nfs4_lookup_revalidate() does check at the moment.

One question is whether this can be done without dropping out of RCU
mode, which might be a real performance win. I'm not sure about
dereferencing inode->i_mode.  AFAICS it should be fine, considering
that destruction of the inode will leave the mode bits untouched,
but...

Thanks,
Miklos
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