linux-kernel - Re: [PATCH 00/10] Fix confusion around MAX

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Date:   Thu, 28 Sep 2023 09:50:07 +0200 (CEST)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>, quic_jhugo@...cinc.com,
        snitzer@...nel.org, dm <dm-devel@...hat.com>, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/10] Fix confusion around MAX_ORDER



On Wed, 27 Sep 2023, Paolo Bonzini wrote:

> On 3/15/23 12:31, 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.
> > 
> > Fix the bugs and then change the definition of MAX_ORDER to be
> > inclusive: the range of orders user can ask from buddy allocator is
> > 0..MAX_ORDER now.

I think that exclusive MAX_ORDER is more intuitive in the C language - 
i.e. if you write "for (i = 0; i < MAX_ORDER; i++)", you are supposed to 
loop over all allowed values. If you declare an array "void 
*array[MAX_ORDER];" you are supposed to hold a value for each allowed 
order.

Pascal has for loops and array dimensions with inclusive ranges - and it 
is more prone to off-by-one errors.

Mikulas