Re: [PATCH 1/3] mm: hugetlbfs: fix hugetlbfs optimization

Andrew Morton
2013-11-19T23:11:49Z

On Fri, 15 Nov 2013 18:47:46 +0100 Andrea Arcangeli <aarcange@redhat.com> wrote:

> The patch from commit 7cb2ef56e6a8b7b368b2e883a0a47d02fed66911 can
> cause dereference of a dangling pointer if split_huge_page runs during
> PageHuge() if there are updates to the tail_page->private field.
>
> Also it is repeating compound_head twice for hugetlbfs and it is
> running compound_head+compound_trans_head for THP when a single one is
> needed in both cases.
>
> The new code within the PageSlab() check doesn't need to verify that
> the THP page size is never bigger than the smallest hugetlbfs page
> size, to avoid memory corruption.
>
> A longstanding theoretical race condition was found while fixing the
> above (see the change right after the skip_unlock label, that is
> relevant for the compound_lock path too).
>
> By re-establishing the _mapcount tail refcounting for all compound
> pages, this also fixes the below problem:
>
> echo 0 >/sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
>
> ...
>
> +/*
> + * PageHeadHuge() only returns true for hugetlbfs head page, but not for
> + * normal or transparent huge pages.
> + */
> +int PageHeadHuge(struct page *page_head)
> +{
> + compound_page_dtor *dtor;
> +
> + if (!PageHead(page_head))
> + return 0;
> +
> + dtor = get_compound_page_dtor(page_head);
> +
> + return dtor == free_huge_page;
> +}
> +EXPORT_SYMBOL_GPL(PageHeadHuge);

This is all rather verbose. How about we do this?

--- a/mm/hugetlb.c~mm-hugetlbc-simplify-pageheadhuge-and-pagehuge
+++ a/mm/hugetlb.c
@@ -690,15 +690,11 @@ static void prep_compound_gigantic_page(
*/
int PageHuge(struct page *page)
{
- compound_page_dtor *dtor;
-
if (!PageCompound(page))
return 0;

page = compound_head(page);
- dtor = get_compound_page_dtor(page);
-
- return dtor == free_huge_page;
+ return get_compound_page_dtor(page) == free_huge_page;
}
EXPORT_SYMBOL_GPL(PageHuge);

@@ -708,14 +704,10 @@ EXPORT_SYMBOL_GPL(PageHuge);
*/
int PageHeadHuge(struct page *page_head)
{
- compound_page_dtor *dtor;
-
if (!PageHead(page_head))
return 0;

- dtor = get_compound_page_dtor(page_head);
-
- return dtor == free_huge_page;
+ return get_compound_page_dtor(page_head) == free_huge_page;
}
EXPORT_SYMBOL_GPL(PageHeadHuge);

> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -82,19 +82,6 @@ static void __put_compound_page(struct page *page)
>
> static void put_compound_page(struct page *page)

This function has become quite crazy. I sat down to refamiliarize but
immediately failed.

: static void put_compound_page(struct page *page)
: {
: if (unlikely(PageTail(page))) {
: ...
: } else if (put_page_testzero(page)) {
: if (PageHead(page))

How can a page be both PageTail() and PageHead()?

: __put_compound_page(page);
: else
: __put_single_page(page);
: }
: }