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Message-ID: <1A762646-9334-48ED-9298-FABA90CA146F@fb.com>
Date: Wed, 6 Nov 2024 22:10:39 +0000
From: Song Liu <songliubraving@...a.com>
To: Amir Goldstein <amir73il@...il.com>
CC: Song Liu <songliubraving@...a.com>, Jeff Layton <jlayton@...nel.org>,
Song
Liu <song@...nel.org>, bpf <bpf@...r.kernel.org>,
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Subject: Re: [RFC bpf-next fanotify 2/5] samples/fanotify: Add a sample
fanotify fastpath handler
Hi Amir,
> On Nov 6, 2024, at 11:40 AM, Amir Goldstein <amir73il@...il.com> wrote:
>
> On Thu, Oct 31, 2024 at 2:52 AM Song Liu <songliubraving@...a.com> wrote:
>>
>> Hi Jeff,
>>
>>> On Oct 30, 2024, at 5:23 PM, Jeff Layton <jlayton@...nel.org> wrote:
>>
>> [...]
>>
>>>> If the subtree is all in the same file system, we can attach fanotify to
>>>> the whole file system, and then use some dget_parent() and follow_up()
>>>> to walk up the directory tree in the fastpath handler. However, if there
>>>> are other mount points in the subtree, we will need more logic to handle
>>>> these mount points.
>>>>
>>>
>>> My 2 cents...
>>>
>>> I'd just confine it to a single vfsmount. If you want to monitor in
>>> several submounts, then you need to add new fanotify watches.
>>>
>
> Agreed.
>
>>> Alternately, maybe there is some way to designate that an entire
>>> vfsmount is a child of a watched (or ignored) directory?
>>>
>>>> @Christian, I would like to know your thoughts on this (walking up the
>>>> directory tree in fanotify fastpath handler). It can be expensive for
>>>> very very deep subtree.
>>>>
>>>
>>> I'm not Christian, but I'll make the case for it. It's basically a
>>> bunch of pointer chasing. That's probably not "cheap", but if you can
>>> do it under RCU it might not be too awful. It might still suck with
>>> really deep paths, but this is a sample module. It's not expected that
>>> everyone will want to use it anyway.
>>
>> Thanks for the suggestion! I will try to do it under RCU.
>>
>
> That's the cost of doing a subtree filter.
> Not sure how it could be avoided?
[...]
>>
>>>
>>> Then, when you get a dentry-based event, you just walk the d_parent
>>> pointers back up to the root of the vfsmount. If one of them matches
>>> the dentry in your fd then you're done. If you hit the root of the
>>> vfsmount, then you're also done (and know that it's not a child of that
>>> dentry).
Thanks for your feedback. I will update the set based on Jeff's suggestion.
Song
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