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

From: Andrew Morton <akpm@linux-foundation.org>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Arun KS <arunks@codeaurora.org>,
	Oscar Salvador <osalvador@suse.de>,
	Michal Hocko <mhocko@suse.com>,
	Pavel Tatashin <pasha.tatashin@soleen.com>,
	Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH v2 4/5] mm/memory_hotplug: Make sure the pfn is aligned to the order when onlining
Date: Wed, 14 Aug 2019 13:56:08 -0700	[thread overview]
Message-ID: <20190814135608.a449ca5a75cd700e077a8d23@linux-foundation.org> (raw)
In-Reply-To: <20190814154109.3448-5-david@redhat.com>

On Wed, 14 Aug 2019 17:41:08 +0200 David Hildenbrand <david@redhat.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.