linux-kernel - Re: [PATCH 10/10] mm, treewide: Redefine MAX

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Date:   Thu, 16 Mar 2023 16:00:03 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/10] mm, treewide: Redefine MAX_ORDER sanely

On 03/16/23 11:15, Mike Kravetz wrote:
> On 03/15/23 14:31, Kirill A. Shutemov wrote:
> > MAX_ORDER currently defined as number of orders page allocator supports:
> > user can ask buddy allocator for page order between 0 and MAX_ORDER-1.
> > 
> > This definition is counter-intuitive and lead to number of bugs all over
> > the kernel.
> > 
> > Change the definition of MAX_ORDER to be inclusive: the range of orders
> > user can ask from buddy allocator is 0..MAX_ORDER now.
> > 
> > --- a/arch/arc/Kconfig
> > +++ b/arch/arc/Kconfig
> > @@ -556,7 +556,7 @@ endmenu	 # "ARC Architecture Configuration"
> >  
> >  config ARCH_FORCE_MAX_ORDER
> >  	int "Maximum zone order"
> > -	default "12" if ARC_HUGEPAGE_16M
> > -	default "11"
> > +	default "11" if ARC_HUGEPAGE_16M
> > +	default "10"
> 
> Is this Kconfig file wrong (off by 1) today?  It seems like it wants MAX_ORDER
> to be sufficiently large to allocate 16M if ARC_HUGEPAGE_16M.  So, seems like
> it should be 13 today?

Nevermind, PAGE_SIZE seems to be 8K by default although it appears to be
configurable.  Not exactly sure how the hard coded values above work if
PAGE_SIZE is configurable.  Too confusing for me to make an intelligent
comment.  Sorry for the noise.
-- 
Mike Kravetz