Re: [Qemu-devel] [PATCH] mm: hugetlb: initialize PG_reserved for tail pages of gigantig compound pages

[Rafael Aquini]

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 <address@hidden>
> Signed-off-by: Andrea Arcangeli <address@hidden>
> ---

Acked-by: Rafael Aquini <address@hidden>


>  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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