mm/hugetlb: fix memfd_pin_folios free_huge_pages leak

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@oracle.com
Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/gup.c b/mm/gup.c
index 8232c8c9c372b59778f6f37d8b4ee9582dcf3b8c..3e7eac9a4f2b4c4536bbe46bd9da496f01195c10 100644 (file)
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -3615,7 +3615,7 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
        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)
@@ -3659,6 +3659,8 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
                                                             &fbatch);
                        if (folio) {
                                folio_put(folio);
+                               if (h)
+                                       folio_put(folio);
                                folio = NULL;
                        }