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Date:   Fri, 15 Jan 2021 11:52:39 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Alexander Potapenko <glider@google.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
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        Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Linux Memory Management List <linux-mm@kvack.org>
Subject: Re: [PATCH v2 3/5] docs: ABI: add /sys/kernel/error_report/
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On Fri, 15 Jan 2021 16:26:21 +0100
Alexander Potapenko <glider@google.com> wrote:

> > please put something like this in
> > tracefs, as there is no such rules there.  Or debugfs, but please, not
> > sysfs.  
> Does tracefs have anything similar to sysfs_notify() or any other way
> to implement a poll() handler?
> Our main goal is to let users wait on poll(), so that they don't have
> to check the file for new contents every now and then. Is it possible
> with tracefs or debugfs?

Polling should work on tracefs. I hope it does, as I depend on it ;-)

And if there's an issue, we can always add more features.

> 
> Also, for our goal debugfs isn't a particularly good fit, as Android
> kernels do not enable debugfs.
> Not sure about tracefs, they seem to have it, need to check.

I believe tracefs is used extensively on Android.

> 
> Do you think it is viable to keep
> /sys/kernel/error_report/report_count in sysfs and use it for
> notifications, but move last_report somewhere else?
> (I'd probably prefer procfs, but it could be tracefs as well, if you
> find that better).

If you do use tracefs, add it to the top level tracing directory (no need
to have instances of it), and rename it to "kernel_warnings", as
"error_report" is too close to the existing "error_log" which holds error
messages of syntactic errors done by users entering in commands to some of
the special files.

That is, /sys/kernel/tracing/kernel_warnings/ would be your error_report/
directory you have now.

Use the function in kernel/trace/trace.c: tracer_init_tracefs() to add that
directory. That's for files in the tracefs directory that will not be
duplicated by instances.

-- Steve