linux-kernel - Re: code style: Re: [PATCH v4] printk: Userspace format enumeration support

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Date:   Wed, 17 Feb 2021 17:09:41 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Chris Down <chris@...isdown.name>
Cc:     linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        John Ogness <john.ogness@...utronix.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>, kernel-team@...com
Subject: Re: code style: Re: [PATCH v4] printk: Userspace format enumeration
 support

On Tue 2021-02-16 17:27:08, Chris Down wrote:
> Petr Mladek writes:
> > > +/*
> > > + * Stores .printk_fmt section boundaries for vmlinux and all loaded modules.
> > > + * Add entries with store_printk_fmt_sec, remove entries with
> > > + * remove_printk_fmt_sec.
> > > + */
> > > +static DEFINE_HASHTABLE(printk_fmts_mod_sections, 8);
> 
> > The hash table looks like an overkill. This is slow path. There are
> > typically only tens of loaded modules. Even the module loader
> > uses plain list for iterating the list of modules.
> 
> I don't think it's overkill -- we have prod systems with hundreds. Hell,
> even my laptop has over 150 loaded. If someone needs to walk all of the
> files in debugfs, it seems unreasonable to do an O(n^2) walk when an O(n)
> one would suffice.
> 
> Unless you have a practical concern, I think this is a distinct case from
> the module loader with its own unique requirements, so I'd prefer to use the
> hash table.

OK, it is true that the module API is either called with a particular
struct module pointer. Or it has to iterate over all modules anyway,
for example, when looking for a symbol.

Well, do we need access to struct module at all?

What about storing the pointer to struct pf_object into
struct printk_fmt_sec *ps into the s->file->f_inode->i_private?
Then we would not need any global list/table at all.

> > > +
> > > +/* Protects printk_fmts_mod_sections */
> > > +static DEFINE_MUTEX(printk_fmts_mutex);
> > 
> > What is the rule for using "printk_fmts" and "printk_fmt", please?
> > I can't find the system here ;-)
> > 
> > Anyway, I would prefer to use "printk_fmt" everywhere.
> > Or maybe even "pf_".
> 
> pf_ sounds fine. :-)
> 
> > > +
> > > +static const char *ps_get_module_name(const struct printk_fmt_sec *ps);
> > > +static int debugfs_pf_open(struct inode *inode, struct file *file);
> > 
> > There are used many different:
> > 
> >   + shortcuts: fmt, fmts, ps, fmt_sec, dfs
> > 
> >   + styles/ordering: ps_get_module_name() vs.
> > 		      find_printk_fmt_sec() vs.
> > 		      printk_fmt_size() vs.
> > 		      debugfs_pf_open()
> > 
> > It might be bearable because there is not much code. But it would
> > still help a lot when we make it more consistent. Many subsystems
> > prefer using a feature-specific prefix.
> > 
> > It might be "printk_fmt_" or "pf_" [*] in this case. And we could use
> > names like:
> > 
> > 	+ struct pf_object [**]
> > 	+ pf_get_object_name()
> > 	+ pf_find_object()
> > 	+ pf_fops,
> > 	+ pf_open()
> > 	+ pf_release()
> > 	+ pf_mutex,
> > 	+ pf_add_object()
> > 	+ pf_remove_object()
> > 	+ pf_module_notify
> 
> Oh, I meant to change the name for v4 but neglected to do so. Sounds good,
> will do.

Thanks a lot. I am sorry that I ask you to do so many changes.
I talked about the style early enough to make the review easy.
Also I think that it is not ideal and annoing to do these
mass changes and refactoring when the code is already reviewed,
tested, and functional.

Best Regards,
Petr