[PATCH V1 3/5] mm/hugetlb: fix memfd_pin_folios resv_huge_pages leak

[Steve Sistare <steven.sistare@xxxxxxxxxx>]

memfd_pin_folios followed by unpin_folios leaves resv_huge_pages elevated
if the pages were not already faulted in.  During a normal page fault,
resv_huge_pages is consumed here:

hugetlb_fault()
  alloc_hugetlb_folio()
    dequeue_hugetlb_folio_vma()
      dequeue_hugetlb_folio_nodemask()
        dequeue_hugetlb_folio_node_exact()
          free_huge_pages--
      resv_huge_pages--

During memfd_pin_folios, the page is created by calling
alloc_hugetlb_folio_nodemask instead of alloc_hugetlb_folio, and
resv_huge_pages is not modified:

memfd_alloc_folio()
  alloc_hugetlb_folio_nodemask()
    dequeue_hugetlb_folio_nodemask()
      dequeue_hugetlb_folio_node_exact()
        free_huge_pages--

alloc_hugetlb_folio_nodemask has other callers that must not modify
resv_huge_pages.  Therefore, to fix, define an alternate version of
alloc_hugetlb_folio_nodemask for this call site that adjusts
resv_huge_pages.

Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")

Signed-off-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
---
 include/linux/hugetlb.h | 10 ++++++++++
 mm/hugetlb.c            | 17 +++++++++++++++++
 mm/memfd.c              |  9 ++++-----
 3 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 45bf05a..3ddd69b 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -695,6 +695,9 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 struct folio *alloc_hugetlb_folio_nodemask(struct hstate *h, int preferred_nid,
 				nodemask_t *nmask, gfp_t gfp_mask,
 				bool allow_alloc_fallback);
+struct folio *alloc_hugetlb_folio_reserve(struct hstate *h, int preferred_nid,
+					  nodemask_t *nmask, gfp_t gfp_mask);
+
 int hugetlb_add_to_page_cache(struct folio *folio, struct address_space *mapping,
 			pgoff_t idx);
 void restore_reserve_on_error(struct hstate *h, struct vm_area_struct *vma,
@@ -1062,6 +1065,13 @@ static inline struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 }
 
 static inline struct folio *
+alloc_hugetlb_folio_reserve(struct hstate *h, int preferred_nid,
+			    nodemask_t *nmask, gfp_t gfp_mask)
+{
+	return NULL;
+}
+
+static inline struct folio *
 alloc_hugetlb_folio_nodemask(struct hstate *h, int preferred_nid,
 			nodemask_t *nmask, gfp_t gfp_mask,
 			bool allow_alloc_fallback)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index aaf508b..c2d44a1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2564,6 +2564,23 @@ struct folio *alloc_buddy_hugetlb_folio_with_mpol(struct hstate *h,
 	return folio;
 }
 
+struct folio *alloc_hugetlb_folio_reserve(struct hstate *h, int preferred_nid,
+		nodemask_t *nmask, gfp_t gfp_mask)
+{
+	struct folio *folio;
+
+	spin_lock_irq(&hugetlb_lock);
+	folio = dequeue_hugetlb_folio_nodemask(h, gfp_mask, preferred_nid,
+					       nmask);
+	if (folio) {
+		VM_BUG_ON(!h->resv_huge_pages);
+		h->resv_huge_pages--;
+	}
+
+	spin_unlock_irq(&hugetlb_lock);
+	return folio;
+}
+
 /* folio migration callback function */
 struct folio *alloc_hugetlb_folio_nodemask(struct hstate *h, int preferred_nid,
 		nodemask_t *nmask, gfp_t gfp_mask, bool allow_alloc_fallback)
diff --git a/mm/memfd.c b/mm/memfd.c
index e7b7c52..bfe0e71 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -82,11 +82,10 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
 		gfp_mask = htlb_alloc_mask(hstate_file(memfd));
 		gfp_mask &= ~(__GFP_HIGHMEM | __GFP_MOVABLE);
 
-		folio = alloc_hugetlb_folio_nodemask(hstate_file(memfd),
-						     numa_node_id(),
-						     NULL,
-						     gfp_mask,
-						     false);
+		folio = alloc_hugetlb_folio_reserve(hstate_file(memfd),
+						    numa_node_id(),
+						    NULL,
+						    gfp_mask);
 		if (folio && folio_try_get(folio)) {
 			err = hugetlb_add_to_page_cache(folio,
 							memfd->f_mapping,
-- 
1.8.3.1