Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 6 Apr 2023 15:44:23 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Mike Rapoport <rppt@kernel.org>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Guenter Roeck <linux@roeck-us.net>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm-treewide-redefine-max_order-sanely-fix.txt
Message-Id: <20230406154423.20a991bbdd47630fc38d94e8@linux-foundation.org>
In-Reply-To: <ZC82N4sP5xE63kl4@kernel.org>
References: <9460377a-38aa-4f39-ad57-fb73725f92db@roeck-us.net>
        <20230406072529.vupqyrzqnhyozeyh@box.shutemov.name>
        <83e6bc46-dfc0-0e95-e69c-5b996af1e50b@roeck-us.net>
        <20230406151015.yndcm24fyxitvqyc@box.shutemov.name>
        <ZC82N4sP5xE63kl4@kernel.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Fri, 7 Apr 2023 00:14:31 +0300 Mike Rapoport <rppt@kernel.org> wrote:

> > > Shouldn't that be
> > > 		else
> > > 			order = 0;
> > > ?
> > 
> > +Mike.
> > 
> > No. start == 0 is MAX_ORDER-aligned. We want to free the pages in the
> > largest chunks alignment allows.
> 
> Right. Before the changes to MAX_ORDER it was
> 
> 		order = min(MAX_ORDER - 1UL, __ffs(start));
> 
> which would evaluate to 10.
> 
> I'd just prefer the comment to include the explanation about why we choose
> MAX_ORDER for start == 0. Say
> 
> 	/*
> 	 * __ffs() behaviour is undefined for 0 and we want to free the
> 	 * pages in the largest chunks alignment allows, so set order to
> 	 * MAX_ORDER when start == 0
> 	 */

Meanwhile I'd like to fix "various boot failures (hang) on arm targets"
in -next, so I queued up Kirill's informal fix for now.