linux-kernel - Re: [RFC v2 bpf-next 0/9] mm/bpf/perf: Store build id in inode object

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Date:   Wed, 1 Mar 2023 09:07:14 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Hao Luo <haoluo@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Matthew Wilcox <willy@...radead.org>, bpf@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Namhyung Kim <namhyung@...il.com>
Subject: Re: [RFC v2 bpf-next 0/9] mm/bpf/perf: Store build id in inode object

On Tue, Feb 28, 2023 at 10:31:57AM +0100, Jiri Olsa wrote:
> hi,
> this is RFC patchset for adding build id under inode's object.
> 
> The main change to previous post [1] is to use inode object instead of file
> object for build id data.

Please explain what a "build id" is, the use case for it, why we
need to store it in VFS objects, what threat model it is protecting
the system against, etc.

> 
> However.. ;-) while using inode as build id storage place saves some memory
> by keeping just one copy of the build id for all file instances, there seems
> to be another problem.

Yes, the problem being that we can cache hundreds of millions of
inodes in memory, and only a very small subset of them are going to
have open files associated with them. And an even smaller subset are
going to be mmapped.

So, in reality, this proposal won't save any memory at all - it
costs memory for every inode that is not currently being used as
a mmapped elf executable, right?

> The problem is that we read the build id when the file is mmap-ed.

Why? I'm completely clueless as to what this thing does or how it's
used....

> Which is fine for our use case,

Which is?

-Dave.
-- 
Dave Chinner
david@...morbit.com