linux-kernel - Re: [PATCH] mm/filemap: add static for function __add_to_page_cache

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Date:   Wed, 9 Dec 2020 19:02:11 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Stanislaw Gruszka <stf_xl@...pl>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Kubecek <mkubecek@...e.cz>,
        Justin Forbes <jmforbes@...uxtx.org>,
        bpf <bpf@...r.kernel.org>, Alex Shi <alex.shi@...ux.alibaba.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Josef Bacik <josef@...icpanda.com>
Subject: Re: [PATCH] mm/filemap: add static for function __add_to_page_cache_locked

On Wed, Dec 9, 2020 at 6:31 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 9 Dec 2020 17:12:43 -0800
> Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
>
> > > > > > FWIW, I intend to do some consolidation/renaming in this area.  I
> > > > > > trust that will not be a problem?
> > > > >
> > > > > If it does not break anything, it will be not a problem ;-)
> > > > >
> > > > > It's possible that __add_to_page_cache_locked() can be a global symbol
> > > > > with add_to_page_cache_lru() + add_to_page_cache_locked() being just
> > > > > static/inline wrappers around it.
> > > >
> > > > So what happens to BTF if we change this area entirely?  Your IDs
> > > > sound like some kind of ABI to me, which is extremely scary.
> > >
> > > Is BTF becoming the new tracepoint? That is, random additions of things like:
> > >
> > >    BTF_ID(func,__add_to_page_cache_locked)
> > >
> > > Like was done in commit 1e6c62a882155 ("bpf: Introduce sleepable BPF
> > > programs") without any notification to the maintainers of the
> > > __add_to_page_cache_locked code, will suddenly become an API?
> >
> > huh? what api/abi you're talking about?
>
> If the function __add_to_page_cache_locked were to be removed due to
> the code being rewritten,  would it break any user space? If not, then
> there's nothing to worry about. ;-)

That function is marked with ALLOW_ERROR_INJECTION.
So any script that exercises it via debugfs (or via bpf) will not work.
That's nothing new. Same "breakage" happens with kprobes, etc.
The function was marked with error_inject for a reason though.
The refactoring or renaming of this code ideally should provide a way to do
similar pattern of injecting errors in this code path.
It could be a completely new function, of course.