[PATCH 6.11 481/558] mm/hugetlb: fix memfd_pin_folios free_huge_pages leak

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6.11-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Steve Sistare <steven.sistare@xxxxxxxxxx>

commit c56b6f3d801d7ec8965993342bdd9e2972b6cb8e upstream.

memfd_pin_folios followed by unpin_folios fails to restore free_huge_pages
if the pages were not already faulted in, because the folio refcount for
pages created by memfd_alloc_folio never goes to 0.  memfd_pin_folios
needs another folio_put to undo the folio_try_get below:

memfd_alloc_folio()
  alloc_hugetlb_folio_nodemask()
    dequeue_hugetlb_folio_nodemask()
      dequeue_hugetlb_folio_node_exact()
        folio_ref_unfreeze(folio, 1);    ; adds 1 refcount
  folio_try_get()                        ; adds 1 refcount
  hugetlb_add_to_page_cache()            ; adds 512 refcount (on x86)

With the fix, after memfd_pin_folios + unpin_folios, the refcount for the
(unfaulted) page is 512, which is correct, as the refcount for a faulted
unpinned page is 513.

Link: https://lkml.kernel.org/r/1725373521-451395-3-git-send-email-steven.sistare@xxxxxxxxxx
Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
Signed-off-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
Acked-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 mm/gup.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/mm/gup.c
+++ b/mm/gup.c
@@ -3618,7 +3618,7 @@ long memfd_pin_folios(struct file *memfd
 	pgoff_t start_idx, end_idx, next_idx;
 	struct folio *folio = NULL;
 	struct folio_batch fbatch;
-	struct hstate *h;
+	struct hstate *h = NULL;
 	long ret = -EINVAL;
 
 	if (start < 0 || start > end || !max_folios)
@@ -3662,6 +3662,8 @@ long memfd_pin_folios(struct file *memfd
 							     &fbatch);
 			if (folio) {
 				folio_put(folio);
+				if (h)
+					folio_put(folio);
 				folio = NULL;
 			}
 






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