linux-kernel - Re: [PATCH v2 1/8] lib/printbuf: New data structure for heap-allocated strings

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Date:   Mon, 25 Apr 2022 03:44:34 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, hch@....de, hannes@...xchg.org,
        akpm@...ux-foundation.org, linux-clk@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-input@...r.kernel.org,
        roman.gushchin@...ux.dev
Subject: Re: [PATCH v2 1/8] lib/printbuf: New data structure for
 heap-allocated strings

On Sun, Apr 24, 2022 at 04:46:03PM -0700, Joe Perches wrote:
> > + * pr_human_readable_u64, pr_human_readable_s64: Print an integer with human
> > + * readable units.
> 
> Why not extend vsprintf for this using something like %pH[8|16|32|64] 
> or %pH[c|s|l|ll|uc|us|ul|ull] ?

The %pX extension we have is _cute_, but ultimately a bad idea.  It
centralises all kinds of unrelated things in vsprintf.c, eg bdev_name()
and clock() and ip_addr_string().

Really, it's working around that we don't have something like Java's
StringBuffer (which I see both seq_buf and printbuf as attempting to
be).  So we have this primitive format string hack instead of exposing
methods like:

void dentry_string(struct strbuf *, struct dentry *);

as an example,
                if (unlikely(ino == dir->i_ino)) {
                        EXT4_ERROR_INODE(dir, "'%pd' linked to parent dir",
                                         dentry);
                        return ERR_PTR(-EFSCORRUPTED);
                }

would become something like:

		if (unlikely(ino == dir->i_ino)) {
			struct strbuf strbuf;
			strbuf_char(strbuf, '\'');
			dentry_string(strbuf, dentry);
			strbuf_string(strbuf, "' linked to parent dir");
			EXT4_ERROR_INODE(dir, strbuf);
			return ERR_PTR(-EFSCORRUPTED);
		}

which isn't terribly nice, but C has sucky syntax for string
construction.  Other languages have done this better, including Rust.