On Thu, Oct 10, 2013 at 06:12:41PM +0200, Andrea Arcangeli wrote:
> 11feeb498086a3a5907b8148bdf1786a9b18fc55 introduced a memory leak when
> KVM is run on gigantic compound pages.
>
> 11feeb498086a3a5907b8148bdf1786a9b18fc55 depends on the assumption
> that PG_reserved is identical for all head and tail pages of a
> compound page. So that if get_user_pages returns a tail page, we don't
> need to check the head page in order to know if we deal with a
> reserved page that requires different refcounting.
>
> The assumption that PG_reserved is the same for head and tail pages is
> certainly correct for THP and regular hugepages, but gigantic
> hugepages allocated through bootmem don't clear the PG_reserved on the
> tail pages (the clearing of PG_reserved is done later only if the
> gigantic hugepage is freed).
>
> This patch corrects the gigantic compound page initialization so that
> we can retain the optimization in
> 11feeb498086a3a5907b8148bdf1786a9b18fc55. The cacheline was already
> modified in order to set PG_tail so this won't affect the boot time of
> large memory systems.
>
> Reported-by: andy123 <ajs124.ajs124@xxxxxxxxx>
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> ---
Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>
> mm/hugetlb.c | 18 +++++++++++++++++-
> 1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index b49579c..315450e 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -695,8 +695,24 @@ static void prep_compound_gigantic_page(struct page *page, unsigned long order)
> /* we rely on prep_new_huge_page to set the destructor */
> set_compound_order(page, order);
> __SetPageHead(page);
> + __ClearPageReserved(page);
> for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
> __SetPageTail(p);
> + /*
> + * For gigantic hugepages allocated through bootmem at
> + * boot, it's safer to be consistent with the
> + * not-gigantic hugepages and to clear the PG_reserved
> + * bit from all tail pages too. Otherwse drivers using
> + * get_user_pages() to access tail pages, may get the
> + * reference counting wrong if they see the
> + * PG_reserved bitflag set on a tail page (despite the
> + * head page didn't have PG_reserved set). Enforcing
> + * this consistency between head and tail pages,
> + * allows drivers to optimize away a check on the head
> + * page when they need know if put_page is needed after
> + * get_user_pages() or not.
> + */
> + __ClearPageReserved(p);
> set_page_count(p, 0);
> p->first_page = page;
> }
> @@ -1329,9 +1345,9 @@ static void __init gather_bootmem_prealloc(void)
> #else
> page = virt_to_page(m);
> #endif
> - __ClearPageReserved(page);
> WARN_ON(page_count(page) != 1);
> prep_compound_huge_page(page, h->order);
> + WARN_ON(PageReserved(page));
> prep_new_huge_page(h, page, page_to_nid(page));
> /*
> * If we had gigantic hugepages allocated at boot time, we need
>
> --
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