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Date:   Wed, 9 Dec 2020 17:32:06 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Stanislaw Gruszka <stf_xl@wp.pl>,
        Matthew Wilcox <willy@infradead.org>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Michal Kubecek <mkubecek@suse.cz>,
        Justin Forbes <jmforbes@linuxtx.org>,
        bpf <bpf@vger.kernel.org>, Alex Shi <alex.shi@linux.alibaba.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Souptick Joarder <jrdr.linux@gmail.com>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Josef Bacik <josef@toxicpanda.com>
Subject: Re: [PATCH] mm/filemap: add static for function
 __add_to_page_cache_locked
Message-ID: <20201209223206.GA1935@home.goodmis.org>
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On Wed, Dec 09, 2020 at 06:05:52PM +0000, Christoph Hellwig wrote:
> On Wed, Dec 09, 2020 at 04:51:48PM +0100, Stanislaw Gruszka wrote:
> > On Wed, Dec 09, 2020 at 03:08:26PM +0000, Matthew Wilcox wrote:
> > > On Wed, Dec 09, 2020 at 03:46:28PM +0100, Stanislaw Gruszka wrote:
> > > > At this point of release cycle we should probably go with revert,
> > > > but I think the main problem is that BPF and ERROR_INJECTION use
> > > > function that is not intended to be used externally. For external users
> > > > add_to_page_cache_lru() and add_to_page_cache_locked() are exported
> > > > and I think those should be used (see the patch below).
> > > 
> > > 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?

There's no mention in the change log to why __add_to_page_cache_locked was
added. And interesting enough, __add_to_page_cache_locked is not in any header
file, which is why it was switched to static.

-- Steve