On Mon, Dec 21, 2020 at 6:27 PM Oscar Salvador [off-list ref] wrote:

On Thu, Dec 17, 2020 at 08:12:56PM +0800, Muchun Song wrote:

quoted

In the subsequent patch, we will allocate the vmemmap pages when free
HugeTLB pages. But update_and_free_page() is called from a non-task
context(and hold hugetlb_lock), so we can defer the actual freeing in
a workqueue to prevent from using GFP_ATOMIC to allocate the vmemmap
pages.

I think we would benefit from a more complete changelog, at least I had
to stare at the code for a while in order to grasp what are we trying
to do and the reasons behind.

OK. Will do.

quoted

+static void __free_hugepage(struct hstate *h, struct page *page);
+
+/*
+ * As update_and_free_page() is be called from a non-task context(and hold
+ * hugetlb_lock), we can defer the actual freeing in a workqueue to prevent
+ * use GFP_ATOMIC to allocate a lot of vmemmap pages.

The above implies that update_and_free_page() is __always__ called from a
non-task context, but that is not always the case?

IIUC, here is always the case.

quoted

+static void update_hpage_vmemmap_workfn(struct work_struct *work)
 {
-     int i;
+     struct llist_node *node;
+     struct page *page;

+     node = llist_del_all(&hpage_update_freelist);
+
+     while (node) {
+             page = container_of((struct address_space **)node,
+                                  struct page, mapping);
+             node = node->next;
+             page->mapping = NULL;
+             __free_hugepage(page_hstate(page), page);
+
+             cond_resched();
+     }
+}
+static DECLARE_WORK(hpage_update_work, update_hpage_vmemmap_workfn);

I wonder if this should be moved to hugetlb_vmemmap.c

Maybe I can do a try.

quoted

+/*
+ * This is where the call to allocate vmemmmap pages will be inserted.
+ */

I think this should go in the changelog.

OK. Will do.

quoted

+static void __free_hugepage(struct hstate *h, struct page *page)
+{
+     int i;
+
      for (i = 0; i < pages_per_huge_page(h); i++) {
              page[i].flags &= ~(1 << PG_locked | 1 << PG_error |
                              1 << PG_referenced | 1 << PG_dirty |

@@ -1313,13 +1377,17 @@ static void update_and_free_page(struct hstate *h, struct page *page)
      set_page_refcounted(page);
      if (hstate_is_gigantic(h)) {
              /*
-              * Temporarily drop the hugetlb_lock, because
-              * we might block in free_gigantic_page().
+              * Temporarily drop the hugetlb_lock only when this type of
+              * HugeTLB page does not support vmemmap optimization (which
+              * context do not hold the hugetlb_lock), because we might
+              * block in free_gigantic_page().

"
 /*
  * Temporarily drop the hugetlb_lock, because we might block
  * in free_gigantic_page(). Only drop it in case the vmemmap
  * optimization is disabled, since that context does not hold
  * the lock.
  */
" ?

Thanks a lot.

Oscar Salvador
SUSE L3

-- 
Yours,
Muchun