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Date: Tue, 2 Jun 2009 10:08:14 +0300
Message-ID: <84144f020906020008w54b1c628hc6e41dcddd208f5f@mail.gmail.com>
Subject: Re: [PATCH 04/23] vfs: Introduce infrastructure for revoking a file
From: Pekka Enberg <penberg@cs.helsinki.fi>
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Sender: owner-linux-mm@kvack.org
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Hugh Dickins <hugh@veritas.com>, Tejun Heo <tj@kernel.org>, Alexey Dobriyan <adobriyan@gmail.com>, Linus Torvalds <torvalds@linux-foundation.org>, Alan Cox <alan@lxorguk.ukuu.org.uk>, Greg Kroah-Hartman <gregkh@suse.de>, Nick Piggin <npiggin@suse.de>, Andrew Morton <akpm@linux-foundation.org>, Christoph Hellwig <hch@infradead.org>, "Eric W. Biederman" <ebiederm@aristanetworks.com>
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Hi Eric,

On Tue, Jun 2, 2009 at 9:51 AM, Eric W. Biederman <ebiederm@xmission.com> w=
rote:
> Pekka Enberg <penberg@cs.helsinki.fi> writes:
>
>> Hi Eric,
>>
>> On Tue, Jun 2, 2009 at 12:50 AM, Eric W. Biederman
>> <ebiederm@xmission.com> wrote:
>>> +#ifdef CONFIG_FILE_HOTPLUG
>>> +
>>> +static bool file_in_use(struct file *file)
>>> +{
>>> + =A0 =A0 =A0 struct task_struct *leader, *task;
>>> + =A0 =A0 =A0 bool in_use =3D false;
>>> + =A0 =A0 =A0 int i;
>>> +
>>> + =A0 =A0 =A0 rcu_read_lock();
>>> + =A0 =A0 =A0 do_each_thread(leader, task) {
>>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 for (i =3D 0; i < MAX_FILE_HOTPLUG_LOCK_D=
EPTH; i++) {
>>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 if (task->file_hotplug_lo=
ck[i] =3D=3D file) {
>>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 in_use =
=3D true;
>>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 goto foun=
d;
>>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 }
>>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 }
>>> + =A0 =A0 =A0 } while_each_thread(leader, task);
>>> +found:
>>> + =A0 =A0 =A0 rcu_read_unlock();
>>> + =A0 =A0 =A0 return in_use;
>>> +}
>>
>> This seems rather heavy-weight. If we're going to use this
>> infrastructure for forced unmount, I think this will be a problem.
>
>> Can't we two this in two stages: (1) mark a bit that forces
>> file_hotplug_read_trylock to always fail and (2) block until the last
>> remaining in-kernel file_hotplug_read_unlock() has executed?
>
> Yes there is room for more optimization in the slow path.
> I haven't noticed being a problem yet so I figured I would start
> with stupid and simple.

Yup, just wanted to point it out.

On Tue, Jun 2, 2009 at 9:51 AM, Eric W. Biederman <ebiederm@xmission.com> w=
rote:
> I can easily see two passes. =A0The first setting the flag an calling
> f_op->dead. =A0The second some kind of consolidate walk through the task
> list, allowing checking on multiple files at once.
>
> I'm not ready to consider anything that will add cost to the fast
> path in the file descriptors though.

Makes sense.

                        Pekka

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