linux-kernel - Re: [PATCH v3 3/3] bootconfig: Free xbc_data in xbc_destroy

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Date:   Thu, 16 Sep 2021 09:05:03 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, Linux-MM <linux-mm@...ck.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v3 3/3] bootconfig: Free xbc_data in xbc_destroy_all()

On Wed, 15 Sep 2021 10:23:54 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 15 Sep 2021 22:19:52 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > @@ -810,6 +811,8 @@ void __init xbc_destroy_all(void)
> >   * In error cases, @emsg will be updated with an error message and
> >   * @epos will be updated with the error position which is the byte offset
> >   * of @buf. If the error is not a parser error, @epos will be -1.
> > + * Note that the @buf ownership is transferred, so it will be freed
> > + * in xbc_destroy_all().
> >   */
> >  int __init xbc_init(char *buf, const char **emsg, int *epos)
> >  {
> 
> I hate this "ownership transfer". Looking at the use case here:
> 
> init/main.c:
> 
> 	copy = memblock_alloc(size + 1, SMP_CACHE_BYTES);
> 	if (!copy) {
> 		pr_err("Failed to allocate memory for bootconfig\n");
> 		return;
> 	}
> 
> 	memcpy(copy, data, size);
> 	copy[size] = '\0';
> 
> 	ret = xbc_init(copy, &msg, &pos);
> 	if (ret < 0) {
> 
> Instead of having xbc_init() return the node count on success, how about
> having it allocate the buffer to use and then return it?
> 
> That is, move the:
> 
> 	copy = memblock_alloc(size + 1, SMP_CACHE_BYTES);
> 	if (!copy) {
> 		pr_err("Failed to allocate memory for bootconfig\n");
> 		return;
> 	}
> 
> 	memcpy(copy, data, size);
> 	copy[size] = '\0';
> 
> into xbc_init(), and have data, and size be passed to it.
> 
> Then, have it return the pointer of "copy" or NULL on error?

Thanks for pointing it out, that is also good to me.
Let me update it.

> 
> This will keep the semantics of xbc_* owning the buffer that gets
> freed by the destroy.
> 
> The xbc_init() could also do the pr_info() that prints the bytes and
> node count. There's no other reason to pass that node count to the
> caller, is there?

Ah, it is my policy that the error or information message is shown
by caller (since caller can also ignore that, e.g. passing the
testing data), not from the library code.
I learned that from perf-probe and ftrace, sometimes the library
code reused in unexpected way. So I decided to decouple the
generating error message and showing it.

Thank you,

> 
> -- Steve


-- 
Masami Hiramatsu <mhiramat@...nel.org>