On Mon 26-09-16 19:28:10, Gerald Schaefer wrote:
> In dissolve_free_huge_pages(), free hugepages will be dissolved without
> making sure that there are enough of them left to satisfy hugepage
> reservations.
otherwise a poor process with a reservation might get unexpected SIGBUS,
right?
> Fix this by adding a return value to dissolve_free_huge_pages() and
> checking h->free_huge_pages vs. h->resv_huge_pages. Note that this may
> lead to the situation where dissolve_free_huge_page() returns an error
> and all free hugepages that were dissolved before that error are lost,
> while the memory block still cannot be set offline.
Hmm, OK offline failure is certainly a better option than an application
failure.
> Fixes: c8721bbb ("mm: memory-hotplug: enable memory hotplug to handle hugepage")
> Signed-off-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> include/linux/hugetlb.h | 6 +++---
> mm/hugetlb.c | 26 +++++++++++++++++++++-----
> mm/memory_hotplug.c | 4 +++-
> 3 files changed, 27 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index c26d463..fe99e6f 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -450,8 +450,8 @@ static inline pgoff_t basepage_index(struct page *page)
> return __basepage_index(page);
> }
>
> -extern void dissolve_free_huge_pages(unsigned long start_pfn,
> - unsigned long end_pfn);
> +extern int dissolve_free_huge_pages(unsigned long start_pfn,
> + unsigned long end_pfn);
> static inline bool hugepage_migration_supported(struct hstate *h)
> {
> #ifdef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
> @@ -518,7 +518,7 @@ static inline pgoff_t basepage_index(struct page *page)
> {
> return page->index;
> }
> -#define dissolve_free_huge_pages(s, e) do {} while (0)
> +#define dissolve_free_huge_pages(s, e) 0
> #define hugepage_migration_supported(h) false
>
> static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 603bdd0..91ae1f5 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1437,22 +1437,32 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
>
> /*
> * Dissolve a given free hugepage into free buddy pages. This function does
> - * nothing for in-use (including surplus) hugepages.
> + * nothing for in-use (including surplus) hugepages. Returns -EBUSY if the
> + * number of free hugepages would be reduced below the number of reserved
> + * hugepages.
> */
> -static void dissolve_free_huge_page(struct page *page)
> +static int dissolve_free_huge_page(struct page *page)
> {
> + int rc = 0;
> +
> spin_lock(&hugetlb_lock);
> if (PageHuge(page) && !page_count(page)) {
> struct page *head = compound_head(page);
> struct hstate *h = page_hstate(head);
> int nid = page_to_nid(head);
> + if (h->free_huge_pages - h->resv_huge_pages == 0) {
> + rc = -EBUSY;
> + goto out;
> + }
> list_del(&head->lru);
> h->free_huge_pages--;
> h->free_huge_pages_node[nid]--;
> h->max_huge_pages--;
> update_and_free_page(h, head);
> }
> +out:
> spin_unlock(&hugetlb_lock);
> + return rc;
> }
>
> /*
> @@ -1460,16 +1470,22 @@ static void dissolve_free_huge_page(struct page *page)
> * make specified memory blocks removable from the system.
> * Note that this will dissolve a free gigantic hugepage completely, if any
> * part of it lies within the given range.
> + * Also note that if dissolve_free_huge_page() returns with an error, all
> + * free hugepages that were dissolved before that error are lost.
> */
> -void dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
> +int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
> {
> unsigned long pfn;
> + int rc = 0;
>
> if (!hugepages_supported())
> - return;
> + return rc;
>
> for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << minimum_order)
> - dissolve_free_huge_page(pfn_to_page(pfn));
> + if (rc = dissolve_free_huge_page(pfn_to_page(pfn)))
> + break;
> +
> + return rc;
> }
>
> /*
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index b58906b..13998d9 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1945,7 +1945,9 @@ static int __ref __offline_pages(unsigned long start_pfn,
> * dissolve free hugepages in the memory block before doing offlining
> * actually in order to make hugetlbfs's object counting consistent.
> */
> - dissolve_free_huge_pages(start_pfn, end_pfn);
> + ret = dissolve_free_huge_pages(start_pfn, end_pfn);
> + if (ret)
> + goto failed_removal;
> /* check again */
> offlined_pages = check_pages_isolated(start_pfn, end_pfn);
> if (offlined_pages < 0) {
> --
> 2.8.4
--
Michal Hocko
SUSE Labs
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