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

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

On 09/21/23 17:31, Muchun Song wrote:
> 
> 
> On 2023/9/21 09:12, Mike Kravetz wrote:
> > 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:
> > +/**
> > + * 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)
> 
> How about changing parameter of @restored to a list_head type which
> returns the non-optimized (previously) or vmemmap-restored-sucessful huge
> pages?
> In which case, the caller could traverse this returned list to free
> them first like you have implemented in bulk_vmemmap_restore_enomem(),
> it will be more efficient. The meaning of returned value should also
> be changed accordingly since update_and_free_pages_bulk() wants to
> whether there is a vmemmap-optimized huge page being restored sucessfully
> to determine if it should clear hugetlb flag. So
> hugetlb_vmemmap_restore_folios()
> could return how many huge pages being restored successful, if a negative
> number is returned meaning there is some error in the process of restoring
> of vmemmap.
> 

I had similar thoughts.  An updated patch based on this approach is below.
When creating the patch, I discovered that using the function return code
for both number of vmemmap restored pages as well as error code was
unnecessary.  Just checking !list_empty() of non-optimized pages tells us
if any were restored or could be freed.

From b79f6eeb7a11644830bddfc43d2219c149d26405 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 an error from
hugetlb_vmemmap_restore_folios.  We want to continue making as much
forward progress as possible.  A new routine bulk_vmemmap_restore_error
handles this specific situation.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 mm/hugetlb.c         | 94 +++++++++++++++++++++++++++++++-------------
 mm/hugetlb_vmemmap.c | 36 +++++++++++++++++
 mm/hugetlb_vmemmap.h | 10 +++++
 3 files changed, 112 insertions(+), 28 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 70fedf8682c4..11de3f885065 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1834,50 +1834,88 @@ 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_error(struct hstate *h,
+					struct list_head *list,
+					struct list_head *non_op_folios)
 {
 	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 (!list_empty(non_op_folios)) {
+		/*
+		 * Free any restored hugetlb pages so that restore of the
+		 * entire list can be retried.
+		 * The idea is that in the common case of ENOMEM errors freeing
+		 * hugetlb pages with vmemmap we will free up memory so that we
+		 * can allocate vmemmap for more hugetlb pages.
+		 */
+		list_for_each_entry_safe(folio, t_folio, non_op_folios, lru) {
+			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 have done something to cause
+		 * success (such as freeing some memory).
+		 * If unable to restore a hugetlb 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);
+				cond_resched();
+				break;
+			}
 	}
+}
+
+static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
+{
+	int ret;
+	LIST_HEAD(non_op_folio);
+	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
-	 * 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.
+	 * First allocate required vmemmmap (if necessary) for all folios.
+	 * Carefully handle errors and free up any available hugetlb pages
+	 * in an effort to make forward progress.
 	 */
-	if (clear_dtor) {
+retry:
+	ret = hugetlb_vmemmap_restore_folios(h, list, &non_op_folio);
+	if (ret < 0) {
+		bulk_vmemmap_restore_error(h, list, &non_op_folio);
+		goto retry;
+	}
+
+	/*
+	 * At this point, list should be empty, and there should only be
+	 * pages on the non_op_folio list.  free those entries.  Do note
+	 * that the non_op_folio list could be empty.
+	 */
+	VM_WARN_ON(!list_empty(list));
+	if (!list_empty(&non_op_folio)) {
 		spin_lock_irq(&hugetlb_lock);
-		list_for_each_entry(folio, list, lru)
+		list_for_each_entry(folio, &non_op_folio, lru)
 			__clear_hugetlb_destructor(h, folio);
 		spin_unlock_irq(&hugetlb_lock);
 	}
 
-	/*
-	 * Free folios back to low level allocators.  vmemmap and destructors
-	 * were taken care of above, so update_and_free_hugetlb_folio will
-	 * not need to take hugetlb lock.
-	 */
-	list_for_each_entry_safe(folio, t_folio, list, lru) {
+	list_for_each_entry_safe(folio, t_folio, &non_op_folio, lru) {
 		update_and_free_hugetlb_folio(h, folio, false);
 		cond_resched();
 	}
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 4558b814ffab..f827d4efcf8e 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -480,6 +480,42 @@ 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:			hstate.
+ * @folio_list:		list of folios.
+ * @non_op_list:	Output list of folio for which vmemmap exists.
+ *
+ * Return: %0 if vmemmap exists for all folios on the list and all entries have
+ *		been moved to non_op_list.  If an error is encountered restoring
+ *		vmemmap for ANY folio, an error code will be returned to the
+ *		caller.  Processing en entries stops when the first error is
+ *		encountered.  folios processed before the error with vmemmap
+ *		will reside on the non_op_list.  The folio that experienced the
+ *		error and all non-processed folios will remain on folio_list.
+ */
+int hugetlb_vmemmap_restore_folios(const struct hstate *h,
+					struct list_head *folio_list,
+					struct list_head *non_op_list)
+{
+	struct folio *folio, *t_folio;
+	int ret = 0;
+
+	list_for_each_entry_safe(folio, t_folio, folio_list, lru) {
+		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
+			ret = hugetlb_vmemmap_restore(h, &folio->page);
+			if (ret)
+				goto out;
+		}
+
+		/* Add non-optimized folios to output list */
+		list_move(&folio->lru, non_op_list);
+	}
+
+out:
+	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..e6378ae5c5b6 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -19,6 +19,9 @@
 
 #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,
+					struct list_head *non_op_folios);
 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 +48,13 @@ static inline int hugetlb_vmemmap_restore(const struct hstate *h, struct page *h
 	return 0;
 }
 
+static int hugetlb_vmemmap_restore_folios(const struct hstate *h,
+					struct list_head *folio_list,
+					struct list_head *non_op_folios)
+{
+	return 0;
+}
+
 static inline void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
 {
 }
-- 
2.41.0