Re: [PATCH 15/14] arm: Rename PMD_ORDER to PMD_BITS

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Matthew Wilcox <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>, Mike Rapoport <rppt@kernel.org>,
	linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 15/14] arm: Rename PMD_ORDER to PMD_BITS
Date: Mon, 4 Jul 2022 05:32:33 +0100	[thread overview]
Message-ID: <YsJtYYsB/SinnNzI@shell.armlinux.org.uk> (raw)
In-Reply-To: <YsIQKdYiswzq5kTG@casper.infradead.org>

On Sun, Jul 03, 2022 at 10:54:49PM +0100, Matthew Wilcox wrote:
> On Sun, Jul 03, 2022 at 10:16:45PM +0100, Russell King (Oracle) wrote:
> > On Sun, Jul 03, 2022 at 10:14:41PM +0100, Matthew Wilcox (Oracle) wrote:
> > > This is the number of bits used by a PMD entry, not the order of a PMD.
> > 
> > No, it's not the number of bits. A PMD entry doesn't fit in 2 or 3 bits.
> > This is even more confusing.
> 
> Well, what is it then?  The order of something is PAGE_SIZE << n, and
> that doesn't seem to be what this is.

Where is it defined that "order" means "PAGE_SIZE << n" ?

"order" here is "order of magnitude" and in this case, it is 2^n, just
like order of magnitude in base 10 is 10^n. So strictly, the usage
here is completely correct, but if you describe "order" as "PAGE_SIZE <<
n" that is no longer an order of magnitude, because it doesn't increase
in an order of magnitude (iow, n = 2 isn't PAGE_SIZE * PAGE_SIZE).

Now, if you're trying to tell me that Linux has decided to define
"order" to be something non-standard, I'll accept that, but then we
shouldn't be renaming stuff that is using it in a standard way.

-- 
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