linux-kernel - Re: [PATCH] mm/page_alloc: fix documentation error and remove magic numbers

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Date:   Wed, 24 Jun 2020 10:07:27 -0400
From:   Rafael Aquini <aquini@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Joel Savitz <jsavitz@...hat.com>, linux-kernel@...r.kernel.org,
        Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: [PATCH] mm/page_alloc: fix documentation error and remove magic
 numbers

On Wed, Jun 24, 2020 at 12:12:55PM +0100, Matthew Wilcox wrote:
> On Tue, Jun 23, 2020 at 11:27:12PM -0400, Joel Savitz wrote:
> > In addition, this patch replaces the magic number bounds with symbolic
> > constants to clarify the logic.
> 
> Why do people think this kind of thing makes the code easier to read?
> It actually makes it harder.  Unless the constants are used in more
> than one place, just leave the numbers where they are.
> 
> > @@ -7852,6 +7852,9 @@ void setup_per_zone_wmarks(void)
> >   * 8192MB:	11584k
> >   * 16384MB:	16384k
> >   */
> > +static const int MIN_FREE_KBYTES_LOWER_BOUND = 1 << 7;
> > +static const int MIN_FREE_KBYTES_UPPER_BOUND = 1 << 18;
> > +

I think these constants would look better if declared as an enum.

> >  int __meminit init_per_zone_wmark_min(void)
> >  {
> >  	unsigned long lowmem_kbytes;
> > @@ -7862,10 +7865,10 @@ int __meminit init_per_zone_wmark_min(void)
> >  
> >  	if (new_min_free_kbytes > user_min_free_kbytes) {
> >  		min_free_kbytes = new_min_free_kbytes;
> > -		if (min_free_kbytes < 128)
> > -			min_free_kbytes = 128;
> > -		if (min_free_kbytes > 262144)
> > -			min_free_kbytes = 262144;
> > +		if (min_free_kbytes < MIN_FREE_KBYTES_LOWER_BOUND)
> > +			min_free_kbytes = MIN_FREE_KBYTES_LOWER_BOUND;
> > +		if (min_free_kbytes > MIN_FREE_KBYTES_UPPER_BOUND)
> > +			min_free_kbytes = MIN_FREE_KBYTES_UPPER_BOUND;
> 
> The only thing I'd consider changing there is replacing 262144 with 256
> * 1024.  1 << 18 is not clearer!


>