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    SubjectRe: [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@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.


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