Re: [PATCH v4 4/8] hugetlb: perform vmemmap restoration on a list of pages

[Mike Kravetz <mike.kravetz@oracle.com>]

On 09/20/23 11:03, Muchun Song wrote:
> > On Sep 20, 2023, at 10:56, Muchun Song <muchun.song@linux.dev> wrote:
> >> On Sep 20, 2023, at 04:57, Mike Kravetz <mike.kravetz@oracle.com> wrote:
> >> On 09/19/23 17:52, Muchun Song wrote:
> >>> On 2023/9/19 07:01, Mike Kravetz wrote:
> >>> 
> >>> I still think we should free a non-optimized HugeTLB page if we
> >>> encounter an OOM situation instead of continue to restore
> >>> vemmmap pages. Restoring vmemmmap pages will only aggravate
> >>> the OOM situation. The suitable appraoch is to free a non-optimized
> >>> HugeTLB page to satisfy our allocation of vmemmap pages, what's
> >>> your opinion, Mike?
> >> 
> >> I agree.
> >> 
> >> As you mentioned previously, this may complicate this code path a bit.
> >> I will rewrite to make this happen.
> > 
> > Maybe we could introduced two list passed to update_and_free_pages_bulk (this
> > will be easy for the callers of it), one is for non-optimized huge page,
> > another is optimized one. In update_and_free_pages_bulk, we could first
> > free those non-optimized huge page, and then restore vemmmap pages for
> > those optimized ones, in which case, the code could be simple.
> > hugetlb_vmemmap_restore_folios() dose not need to add complexity, which
> > still continue to restore vmemmap pages and will stop once we encounter
> > an OOM situation.

I am not sure if passing in optimized and non-optimized lists to
update_and_free_pages_bulk will help much.  IIUC, it will almost always
be the case where only one list has entries.  Is that mostly accurate?

> BTW, maybe we should try again iff there are some non-optimized huge page
> whose vmemmap pages are restored successfully previously and could be freed
> first, then continue to restore the vmemmap pages of the remaining huge pages.
> I think the retry code could be done in update_and_free_pages_bulk() as well.

I came up with a new routine to handle these ENOMEM returns from
hugetlb_vmemmap_restore_folios.  I 'think' it handles these situations.
Here is an updated version of this patch.  Sorry, diff makes it a bit
hard to read.

From b13bdccb01730f995191944769f87d0725c289ad Mon Sep 17 00:00:00 2001
From: Mike Kravetz <mike.kravetz@oracle.com>
Date: Sun, 10 Sep 2023 16:14:50 -0700
Subject: [PATCH] hugetlb: perform vmemmap restoration on a list of pages

The routine update_and_free_pages_bulk already performs vmemmap
restoration on the list of hugetlb pages in a separate step.  In
preparation for more functionality to be added in this step, create a
new routine hugetlb_vmemmap_restore_folios() that will restore
vmemmap for a list of folios.

This new routine must provide sufficient feedback about errors and
actual restoration performed so that update_and_free_pages_bulk can
perform optimally.

Special care must be taken when encountering a ENOMEM error from
hugetlb_vmemmap_restore_folios.  We want to continue making as much
forward progress as possible.  A new routine bulk_vmemmap_restore_enomem
handles this specific situation.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 mm/hugetlb.c         | 83 ++++++++++++++++++++++++++++++++++----------
 mm/hugetlb_vmemmap.c | 39 +++++++++++++++++++++
 mm/hugetlb_vmemmap.h | 11 ++++++
 3 files changed, 115 insertions(+), 18 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 70fedf8682c4..52abe56cf38a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1834,38 +1834,85 @@ static void update_and_free_hugetlb_folio(struct hstate *h, struct folio *folio,
 		schedule_work(&free_hpage_work);
 }
 
-static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
+static void bulk_vmemmap_restore_enomem(struct hstate *h,
+						struct list_head *list,
+						unsigned long restored)
 {
 	struct folio *folio, *t_folio;
-	bool clear_dtor = false;
 
-	/*
-	 * First allocate required vmemmmap (if necessary) for all folios on
-	 * list.  If vmemmap can not be allocated, we can not free folio to
-	 * lower level allocator, so add back as hugetlb surplus page.
-	 * add_hugetlb_folio() removes the page from THIS list.
-	 * Use clear_dtor to note if vmemmap was successfully allocated for
-	 * ANY page on the list.
-	 */
-	list_for_each_entry_safe(folio, t_folio, list, lru) {
-		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
+	if (restored) {
+		/*
+		 * On ENOMEM error, free any restored hugetlb pages so that
+		 * restore of the entire list can be retried.
+		 * The idea is that by freeing hugetlb pages with vmemmap
+		 * (those previously restored) we will free up memory so that
+		 * we can allocate vmemmap for more hugetlb pages.
+		 * We must examine and possibly free EVERY hugetlb page on list
+		 * in order to call hugetlb_vmemmap_restore_folios again.
+		 * This is not optimal, but is an error case that should not
+		 * happen frequently.
+		 */
+		list_for_each_entry_safe(folio, t_folio, list, lru)
+			if (!folio_test_hugetlb_vmemmap_optimized(folio)) {
+				list_del(&folio->lru);
+				spin_lock_irq(&hugetlb_lock);
+				__clear_hugetlb_destructor(h, folio);
+				spin_unlock_irq(&hugetlb_lock);
+				update_and_free_hugetlb_folio(h, folio, false);
+				cond_resched();
+			}
+	} else {
+		/*
+		 * In the case where vmemmap was not restored for ANY folios,
+		 * we loop through them trying to restore individually in the
+		 * hope that someone elsewhere may free enough memory.
+		 * If unable to restore a page, the hugetlb page is made a
+		 * surplus page and removed from the list.
+		 * If are able to restore vmemmap for one hugetlb page, we free
+		 * it and quit processing the list to retry the bulk operation.
+		 */
+		list_for_each_entry_safe(folio, t_folio, list, lru)
 			if (hugetlb_vmemmap_restore(h, &folio->page)) {
 				spin_lock_irq(&hugetlb_lock);
 				add_hugetlb_folio(h, folio, true);
 				spin_unlock_irq(&hugetlb_lock);
-			} else
-				clear_dtor = true;
-		}
+			} else {
+				list_del(&folio->lru);
+				spin_lock_irq(&hugetlb_lock);
+				__clear_hugetlb_destructor(h, folio);
+				spin_unlock_irq(&hugetlb_lock);
+				update_and_free_hugetlb_folio(h, folio, false);
+				break;
+			}
 	}
+}
+
+static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
+{
+	int ret;
+	unsigned long restored;
+	struct folio *folio, *t_folio;
 
 	/*
-	 * If vmemmmap allocation was performed on any folio above, take lock
-	 * to clear destructor of all folios on list.  This avoids the need to
+	 * First allocate required vmemmmap (if necessary) for all folios.
+	 * Carefully handle ENOMEM errors and free up any available hugetlb
+	 * pages in order to make forward progress.
+	 */
+retry:
+	ret = hugetlb_vmemmap_restore_folios(h, list, &restored);
+	if (ret == -ENOMEM) {
+		bulk_vmemmap_restore_enomem(h, list, restored);
+		goto retry;
+	}
+
+	/*
+	 * If vmemmmap allocation was performed on ANY folio , take lock to
+	 * clear destructor of all folios on list.  This avoids the need to
 	 * lock/unlock for each individual folio.
 	 * The assumption is vmemmap allocation was performed on all or none
 	 * of the folios on the list.  This is true expect in VERY rare cases.
 	 */
-	if (clear_dtor) {
+	if (restored) {
 		spin_lock_irq(&hugetlb_lock);
 		list_for_each_entry(folio, list, lru)
 			__clear_hugetlb_destructor(h, folio);
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 4558b814ffab..cc91edbfb68b 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -480,6 +480,45 @@ int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head)
 	return ret;
 }
 
+/**
+ * hugetlb_vmemmap_restore_folios - restore vmemmap for every folio on the list.
+ * @h:		struct hstate.
+ * @folio_list:	list of folios.
+ * @restored:	Set to number of folios for which vmemmap was restored
+ *		successfully if caller passes a non-NULL pointer.
+ *
+ * Return: %0 if vmemmap exists for all folios on the list.  If an error is
+ *		encountered restoring vmemmap for ANY folio, an error code
+ *		will be returned to the caller.  It is then the responsibility
+ *		of the caller to check the hugetlb vmemmap optimized flag of
+ *		each folio to determine if vmemmap was actually restored.
+ *		Note that processing is stopped when first error is encountered.
+ */
+int hugetlb_vmemmap_restore_folios(const struct hstate *h,
+					struct list_head *folio_list,
+					unsigned long *restored)
+{
+	unsigned long num_restored;
+	struct folio *folio;
+	int ret = 0;
+
+	num_restored = 0;
+	list_for_each_entry(folio, folio_list, lru) {
+		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
+			ret = hugetlb_vmemmap_restore(h, &folio->page);
+			if (ret)
+				goto out;
+			else
+				num_restored++;
+		}
+	}
+
+out:
+	if (*restored)
+		*restored = num_restored;
+	return ret;
+}
+
 /* Return true iff a HugeTLB whose vmemmap should and can be optimized. */
 static bool vmemmap_should_optimize(const struct hstate *h, const struct page *head)
 {
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index c512e388dbb4..bb58453c3cc0 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -19,6 +19,8 @@
 
 #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
 int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head);
+int hugetlb_vmemmap_restore_folios(const struct hstate *h,
+			struct list_head *folio_list, unsigned long *restored);
 void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head);
 void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_list);
 
@@ -45,6 +47,15 @@ static inline int hugetlb_vmemmap_restore(const struct hstate *h, struct page *h
 	return 0;
 }
 
+static inline int hugetlb_vmemmap_restore_folios(const struct hstate *h,
+					struct list_head *folio_list,
+					unsigned long *restored)
+{
+	if (restored)
+		*restored = 0;
+	return 0;
+}
+
 static inline void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
 {
 }
-- 
2.41.0