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Date:   Wed, 14 Aug 2019 15:10:59 -0700
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
Message-Id: <20190814151059.54618c4f86b7e0c613ab7413@linux-foundation.org>
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        <20190814154109.3448-5-david@redhat.com>
        <20190814135608.a449ca5a75cd700e077a8d23@linux-foundation.org>
        <784cba14-e0ad-cfea-8ffc-bfbf855ceb10@redhat.com>
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On Wed, 14 Aug 2019 23:47:24 +0200 David Hildenbrand <david@redhat.com> wrote:

> >> --- 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.
> > 
> > 
> 
> Makes sense, do you want to add the WARN_ON_ONCE() or shall I resend?

--- a/mm/memory_hotplug.c~mm-memory_hotplug-make-sure-the-pfn-is-aligned-to-the-order-when-onlining-fix
+++ a/mm/memory_hotplug.c
@@ -647,7 +647,7 @@ static int online_pages_range(unsigned l
 	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)))
+		if (WARN_ON_ONCE(!IS_ALIGNED(pfn, 1ul << order)))
 			order = 0;
 		(*online_page_callback)(pfn_to_page(pfn), order);
 	}
_