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Subject: Re: [PATCH v2 4/5] mm/memory_hotplug: Make sure the pfn is aligned to
 the order when onlining
To:     Andrew Morton <akpm@linux-foundation.org>
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>
References: <20190814154109.3448-1-david@redhat.com>
 <20190814154109.3448-5-david@redhat.com>
 <20190814135608.a449ca5a75cd700e077a8d23@linux-foundation.org>
From:   David Hildenbrand <david@redhat.com>
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Message-ID: <784cba14-e0ad-cfea-8ffc-bfbf855ceb10@redhat.com>
Date:   Wed, 14 Aug 2019 23:47:24 +0200
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On 14.08.19 22:56, Andrew Morton wrote:
> 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.
> 
> 

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

I was recently thinking about limiting offlining to memory blocks
without holes - then also onlining would only apply to memory blocks
without holes and we could simplify both paths (single zone/node, no
holes) - including this check, we would always have memory block size
alignments. But I am not sure yet if there is a valid use case for
offlining/re-online boot memory with holes.

-- 

Thanks,

David / dhildenb