linux-kernel - Re: [PATCH v2 4/5] mm/memory_hotplug: Make sure the pfn is aligned to the order when onlining

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Message-Id: <20190814135608.a449ca5a75cd700e077a8d23@linux-foundation.org>
Date:   Wed, 14 Aug 2019 13:56:08 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Arun KS <arunks@...eaurora.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 4/5] mm/memory_hotplug: Make sure the pfn is aligned
 to the order when onlining

On Wed, 14 Aug 2019 17:41:08 +0200 David Hildenbrand <david@...hat.com> wrote:

> Commit a9cd410a3d29 ("mm/page_alloc.c: memory hotplug: free pages as higher
> order") assumed that any PFN we get via memory resources is aligned to
> to MAX_ORDER - 1, I am not convinced that is always true. Let's play safe,
> check the alignment and fallback to single pages.
> 
> ...
>
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -646,6 +646,9 @@ static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
>  	 */
>  	for (pfn = start_pfn; pfn < end_pfn; pfn += 1ul << order) {
>  		order = min(MAX_ORDER - 1, get_order(PFN_PHYS(end_pfn - pfn)));
> +		/* __free_pages_core() wants pfns to be aligned to the order */
> +		if (unlikely(!IS_ALIGNED(pfn, 1ul << order)))
> +			order = 0;
>  		(*online_page_callback)(pfn_to_page(pfn), order);
>  	}

We aren't sure if this occurs, but if it does, we silently handle it.

It seems a reasonable defensive thing to do, but should we add a
WARN_ON_ONCE() so that we get to find out about it?  If we get such a
report then we can remove the WARN_ON_ONCE() and add an illuminating
comment.