linux-kernel - Re: [PATCH V4 01/20] rv: Add Runtime Verification (RV) interface

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Date:   Fri, 8 Jul 2022 23:36:06 +0800
From:   Tao Zhou <tao.zhou@...ux.dev>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Daniel Bristot de Oliveira <bristot@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marco Elver <elver@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Gabriele Paoloni <gpaoloni@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-trace-devel@...r.kernel.org, Tao Zhou <tao.zhou@...ux.dev>
Subject: Re: [PATCH V4 01/20] rv: Add Runtime Verification (RV) interface

On Wed, Jul 06, 2022 at 06:53:21PM +0100, Matthew Wilcox wrote:

> On Thu, Jul 07, 2022 at 01:49:02AM +0800, Tao Zhou wrote:
> > > +struct rv_monitor {
> > > +	const char		*name;
> > > +	const char		*description;
> > > +	bool			enabled;
> > 
> > Can the 'bool enabled;' be put at the end like the definition of
> > structure rv_monitor_def. If '8+8+sizeof(bool)+8+8+8' not the same
> > as '8+8+8+8+8+sizeof(bool)', I mean is it possible that after the
> > end of stucture there is a int or char not require to align to 8 as
> > an example from my nonsense.
> 
> That will make no difference at all.  C doesn't allow other variables
> to "fill in the hole" at the end of the structure like that.  For
> example, one could legitimately do 'memset(&rvm, sizeof(rvm))',
> and that would wipe out those other variables as well.

I mean if it is possible that if @enabled placed at the end of the
structure rv_monitor will save some bytes.
If @enabled place in between, the next function pointer which is 8 bytes
will align to be in x8 address and the size of structure rv_monitor is
larger than been placed at the end of the structure.
Or the compiler can do magic that I can not guess.

Sorry for my late reply. I am not sure about this. But your reply is not
about what I mean. You say that the size of structure is the same(I doute about this).
But what my concert is that the other data next to the structure rv_monitor 
how to align if placing the @enabled at the end.

Place in between, bytes:
8+8+8(padd)+8+8+8=48

Place at the end, bytes:
8+8+8+8+8+sizeof(bool)=?

? is small than 48 and the data next to the data structure rv_monitor can use
the saved byte which is the result of placing @enabled at the end of structure
to place the data.