[PATCH v3] mm: hugetlb: improve the handling of hugetlb allocation failure for freed or in-use hugetlb

[Baolin Wang <baolin.wang@linux.alibaba.com>]

alloc_and_dissolve_hugetlb_folio() preallocates a new hugetlb page before
it takes hugetlb_lock. In 3 out of 4 cases the page is not really used and
therefore the newly allocated page is just freed right away. This is
wasteful and it might cause pre-mature failures in those cases.

Address that by moving the allocation down to the only case (hugetlb
page is really in the free pages pool). We need to drop hugetlb_lock
to do so and therefore need to recheck the page state after regaining
it.

The patch is more of a cleanup than an actual fix to an existing
problem. There are no known reports about pre-mature failures.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
Changes from v2；
 - Update the commit message suggested by Michal.
 - Remove unnecessary comments.
Changes from v1:
 - Update the suject line per Muchun.
 - Move the allocation into the free hugetlb handling branch per Michal.
---
 mm/hugetlb.c | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9d996fe4ecd9..a05507a2143f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3031,21 +3031,9 @@ static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
 {
 	gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
 	int nid = folio_nid(old_folio);
-	struct folio *new_folio;
+	struct folio *new_folio = NULL;
 	int ret = 0;
 
-	/*
-	 * Before dissolving the folio, we need to allocate a new one for the
-	 * pool to remain stable.  Here, we allocate the folio and 'prep' it
-	 * by doing everything but actually updating counters and adding to
-	 * the pool.  This simplifies and let us do most of the processing
-	 * under the lock.
-	 */
-	new_folio = alloc_buddy_hugetlb_folio(h, gfp_mask, nid, NULL, NULL);
-	if (!new_folio)
-		return -ENOMEM;
-	__prep_new_hugetlb_folio(h, new_folio);
-
 retry:
 	spin_lock_irq(&hugetlb_lock);
 	if (!folio_test_hugetlb(old_folio)) {
@@ -3075,6 +3063,16 @@ static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
 		cond_resched();
 		goto retry;
 	} else {
+		if (!new_folio) {
+			spin_unlock_irq(&hugetlb_lock);
+			new_folio = alloc_buddy_hugetlb_folio(h, gfp_mask, nid,
+							      NULL, NULL);
+			if (!new_folio)
+				return -ENOMEM;
+			__prep_new_hugetlb_folio(h, new_folio);
+			goto retry;
+		}
+
 		/*
 		 * Ok, old_folio is still a genuine free hugepage. Remove it from
 		 * the freelist and decrease the counters. These will be
@@ -3102,9 +3100,11 @@ static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
 
 free_new:
 	spin_unlock_irq(&hugetlb_lock);
-	/* Folio has a zero ref count, but needs a ref to be freed */
-	folio_ref_unfreeze(new_folio, 1);
-	update_and_free_hugetlb_folio(h, new_folio, false);
+	if (new_folio) {
+		/* Folio has a zero ref count, but needs a ref to be freed */
+		folio_ref_unfreeze(new_folio, 1);
+		update_and_free_hugetlb_folio(h, new_folio, false);
+	}
 
 	return ret;
 }
-- 
2.39.3