linux-kernel - Re: [PATCH 15/14] arm: Rename PMD_ORDER to PMD

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Date:   Mon, 4 Jul 2022 11:48:39 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, Mike Rapoport <rppt@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 15/14] arm: Rename PMD_ORDER to PMD_BITS

On Mon, Jul 04, 2022 at 05:32:33AM +0100, Russell King (Oracle) wrote:
> 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" ?

include/asm-generic/getorder.h: * get_order - Determine the allocation order of a memory size

> "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.
> 
> -- 
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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