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

[Song Liu <songliubraving@meta.com>]

> On Nov 7, 2024, at 2:41 AM, Jan Kara <jack@suse.cz> wrote:
> 
> On Wed 06-11-24 20:40:50, Amir Goldstein wrote:
>> On Thu, Oct 31, 2024 at 2:52 AM Song Liu <songliubraving@meta.com> wrote:
>>>> 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?
> 
> Yes. For a real solution (not this example), we'd probably have to limit
> the parent walk to say 16 steps and if we can reach neither root nor our
> dir in that number of steps, we'll just chicken out and pass the event to
> userspace to deal with it. This way the kernel filter will deal with most
> cases anyway and we won't risk livelocking or too big performance overhead
> of the filter.
> 
> For this example, I think using fs/dcache.c:is_subdir() will be OK for
> demonstration purposes.

Thanks for the pointer! It does look like a good solution for this example.

Song