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Date:   Fri, 7 Apr 2023 21:03:24 +0300
From:   Mike Rapoport <rppt@kernel.org>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        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
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On Fri, Apr 07, 2023 at 03:40:54PM +0300, Kirill A. Shutemov wrote:
> On Thu, Apr 06, 2023 at 03:44:23PM -0700, Andrew Morton wrote:
> > 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.
> 
> Here's my variant of the fix up with more vervose comments.
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 7911224b1ed3..381e36ac9e4d 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -2043,7 +2043,16 @@ static void __init __free_pages_memory(unsigned long start, unsigned long end)
>  	int order;
>  
>  	while (start < end) {
> -		order = min_t(int, MAX_ORDER, __ffs(start));
> +		/*
> +		 * Free the pages in the largest chunks alignment allows.
> +		 *
> +		 * __ffs() behaviour is undefined for 0. start == 0 is
> +		 * MAX_ORDER-aligned, Set order to MAX_ORDER for the case.

                                      ^ small s
otherwise feel free to add

Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>

> +		 */
> +		if (start)
> +			order = min_t(int, MAX_ORDER, __ffs(start));
> +		else
> +			order = MAX_ORDER;
>  
>  		while (start + (1UL << order) > end)
>  			order--;
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index c8f0a8c2d049..8e0fa209d533 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -605,7 +605,18 @@ static void online_pages_range(unsigned long start_pfn, unsigned long nr_pages)
>  	 * this and the first chunk to online will be pageblock_nr_pages.
>  	 */
>  	for (pfn = start_pfn; pfn < end_pfn;) {
> -		int order = min_t(int, MAX_ORDER, __ffs(pfn));
> +		int order;
> +
> +		/*
> +		 * Free to online pages in the largest chunks alignment allows.
> +		 *
> +		 * __ffs() behaviour is undefined for 0. start == 0 is
> +		 * MAX_ORDER-aligned, Set order to MAX_ORDER for the case.
> +		 */
> +		if (pfn)
> +			order = min_t(int, MAX_ORDER, __ffs(pfn));
> +		else
> +			order = MAX_ORDER;
>  
>  		(*online_page_callback)(pfn_to_page(pfn), order);
>  		pfn += (1UL << order);
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov

-- 
Sincerely yours,
Mike.