linux-kernel - Re: [PATCH v2 1/8] minmax: Put all the clamp() definitions together

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Message-ID: <CAHk-=wigkg43sZJKuRuYOzWzj9bxczE2toHNc=yxfyMM3m_kcA@mail.gmail.com>
Date: Sun, 28 Jul 2024 13:31:47 -0700
From: Linus Torvalds <torvalds@...uxfoundation.org>
To: David Laight <David.Laight@...lab.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, Christoph Hellwig <hch@...radead.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	Dan Carpenter <dan.carpenter@...aro.org>, Arnd Bergmann <arnd@...nel.org>, 
	"Jason@...c4.com" <Jason@...c4.com>, "pedro.falcato@...il.com" <pedro.falcato@...il.com>, 
	Mateusz Guzik <mjguzik@...il.com>, "linux-mm@...ck.org" <linux-mm@...ck.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Subject: Re: [PATCH v2 1/8] minmax: Put all the clamp() definitions together

On Sun, 28 Jul 2024 at 13:23, David Laight <David.Laight@...lab.com> wrote:
>
> MIN() (and probably your MIN_T()) ought to have a check for
> being a constant in order to stop misuse.

No, we have a number of "runtime constants" that are basically
"constants" set up at boot-time for the architecture,as pointed out by
the powerpc people in private:

Ie, we have arch/powerpc/include/asm/page.h:

   #define HPAGE_SHIFT hpage_shift

and then

  #define HUGETLB_PAGE_ORDER      (HPAGE_SHIFT - PAGE_SHIFT)

and then

   #define pageblock_order         MIN_T(unsigned int,
HUGETLB_PAGE_ORDER, MAX_PAGE_ORDER)

and we really *REALLY* don't want to force the complicated "min_t()"
(or, worse yet, "min()") functions here just because there's actually
a variable involved.

That variable gets initialized early in
hugetlbpage_init_defaultsize(), so it's *effectively* a constant, but
not as far as the compiler is concerned.

           Linus