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

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Message-ID: <02D2F126-C40B-4EC1-92E5-243238B348A7@fb.com>
Date: Thu, 7 Nov 2024 19:39:58 +0000
From: Song Liu <songliubraving@...a.com>
To: Jan Kara <jack@...e.cz>
CC: Amir Goldstein <amir73il@...il.com>, Song Liu <songliubraving@...a.com>,
        Jeff Layton <jlayton@...nel.org>, Song Liu <song@...nel.org>,
        bpf
	<bpf@...r.kernel.org>,
        Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML
	<linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...a.com>,
        Andrii
 Nakryiko <andrii@...nel.org>,
        Eduard Zingerman <eddyz87@...il.com>,
        Alexei
 Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin
 KaFai Lau <martin.lau@...ux.dev>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, KP Singh <kpsingh@...nel.org>,
        Matt
 Bobrowski <mattbobrowski@...gle.com>,
        "repnop@...gle.com"
	<repnop@...gle.com>,
        Josef Bacik <josef@...icpanda.com>
Subject: Re: [RFC bpf-next fanotify 2/5] samples/fanotify: Add a sample
 fanotify fastpath handler



> On Nov 7, 2024, at 2:41 AM, Jan Kara <jack@...e.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@...a.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