On 12/04/2017 06:01 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> alloc_surplus_huge_page increases the pool size and the number of
> surplus pages opportunistically to prevent from races with the pool size
> change. See d1c3fb1f8f29 ("hugetlb: introduce nr_overcommit_hugepages
> sysctl") for more details.
>
> The resulting code is unnecessarily hairy, cause code duplication and
> doesn't allow to share the allocation paths. Moreover pool size changes
> tend to be very seldom so optimizing for them is not really reasonable.
> Simplify the code and allow to allocate a fresh surplus page as long as
> we are under the overcommit limit and then recheck the condition after
> the allocation and drop the new page if the situation has changed. This
> should provide a reasonable guarantee that an abrupt allocation requests
> will not go way off the limit.
>
> If we consider races with the pool shrinking and enlarging then we
> should be reasonably safe as well. In the first case we are off by one
> in the worst case and the second case should work OK because the page is
> not yet visible. We can waste CPU cycles for the allocation but that
> should be acceptable for a relatively rare condition.
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/hugetlb.c | 60 +++++++++++++++++++++---------------------------------------
> 1 file changed, 21 insertions(+), 39 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index a1b8b2888ec9..0c7dc269b6c0 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1538,62 +1538,44 @@ int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
> static struct page *__alloc_surplus_huge_page(struct hstate *h, gfp_t gfp_mask,
> int nid, nodemask_t *nmask)
> {
> - struct page *page;
> - unsigned int r_nid;
> + struct page *page = NULL;
>
> if (hstate_is_gigantic(h))
> return NULL;
>
> - /*
> - * Assume we will successfully allocate the surplus page to
> - * prevent racing processes from causing the surplus to exceed
> - * overcommit
> - *
> - * This however introduces a different race, where a process B
> - * tries to grow the static hugepage pool while alloc_pages() is
> - * called by process A. B will only examine the per-node
> - * counters in determining if surplus huge pages can be
> - * converted to normal huge pages in adjust_pool_surplus(). A
> - * won't be able to increment the per-node counter, until the
> - * lock is dropped by B, but B doesn't drop hugetlb_lock until
> - * no more huge pages can be converted from surplus to normal
> - * state (and doesn't try to convert again). Thus, we have a
> - * case where a surplus huge page exists, the pool is grown, and
> - * the surplus huge page still exists after, even though it
> - * should just have been converted to a normal huge page. This
> - * does not leak memory, though, as the hugepage will be freed
> - * once it is out of use. It also does not allow the counters to
> - * go out of whack in adjust_pool_surplus() as we don't modify
> - * the node values until we've gotten the hugepage and only the
> - * per-node value is checked there.
> - */
> spin_lock(&hugetlb_lock);
> - if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages) {
> - spin_unlock(&hugetlb_lock);
> - return NULL;
> - } else {
> - h->nr_huge_pages++;
> - h->surplus_huge_pages++;
> - }
> + if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages)
> + goto out_unlock;
> spin_unlock(&hugetlb_lock);
>
> page = __hugetlb_alloc_buddy_huge_page(h, gfp_mask, nid, nmask);
> + if (!page)
> + goto out_unlock;
>
> spin_lock(&hugetlb_lock);
> - if (page) {
> + /*
> + * We could have raced with the pool size change.
> + * Double check that and simply deallocate the new page
> + * if we would end up overcommiting the surpluses. Abuse
> + * temporary page to workaround the nasty free_huge_page
> + * codeflow
> + */
> + if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages) {
> + SetPageHugeTemporary(page);
> + put_page(page);
> + page = NULL;
> + } else {
> + h->surplus_huge_pages_node[page_to_nid(page)]++;
> + h->surplus_huge_pages++;
> INIT_LIST_HEAD(&page->lru);
> r_nid = page_to_nid(page);
> set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> set_hugetlb_cgroup(page, NULL);
> - /*
> - * We incremented the global counters already
> - */
> h->nr_huge_pages_node[r_nid]++;
> h->surplus_huge_pages_node[r_nid]++;
> - } else {
> - h->nr_huge_pages--;
> - h->surplus_huge_pages--;
In the case of a successful surplus allocation, the following counters
are incremented:
h->surplus_huge_pages_node[page_to_nid(page)]++;
h->surplus_huge_pages++;
h->nr_huge_pages_node[r_nid]++;
h->surplus_huge_pages_node[r_nid]++;
Looks like per-node surplus_huge_pages_node is incremented twice, and
global nr_huge_pages is not incremented at all.
Also, you removed r_nid so I'm guessing this will not compile?
--
Mike Kravetz
> }
> +
> +out_unlock:
> spin_unlock(&hugetlb_lock);
>
> return page;
>
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