Re: [RFC bpf-next fanotify 2/5] samples/fanotify: Add a sample fanotify fastpath handler

[Song Liu <songliubraving@meta.com>]

Hi Amir, 

> On Nov 6, 2024, at 11:40 AM, Amir Goldstein <amir73il@gmail.com> wrote:
> 
> On Thu, Oct 31, 2024 at 2:52 AM Song Liu <songliubraving@meta.com> wrote:
>> 
>> Hi Jeff,
>> 
>>> On Oct 30, 2024, at 5:23 PM, Jeff Layton <jlayton@kernel.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