[PATCH v2 2/2] mm/gup: memfd: stop leaking pinned pages in low memory conditions

[John Hubbard <jhubbard@nvidia.com>]

If check_and_migrate_movable_pages() fails, typically with -ENOMEM, then
memfd_pin_folios() will leave pages pinned. Those are leaked forever,
and are visible to user space as a memory leak.

Fix this by unpinning the folios that try_grab_folio(FOLL_PIN) has
pinned, in such error cases.

Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
Suggested-by: David Hildenbrand <david@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Hugh Dickins <hughd@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Dongwon Kim <dongwon.kim@intel.com>
Cc: Junxiao Chang <junxiao.chang@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 233c284e8e66..dc4906243b97 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -3719,12 +3719,10 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
 		ret = check_and_migrate_movable_folios(nr_folios, folios);
 	} while (ret == -EAGAIN);
 
-	memalloc_pin_restore(flags);
-	return ret ? ret : nr_folios;
 err:
 	memalloc_pin_restore(flags);
-	unpin_folios(folios, nr_folios);
-
-	return ret;
+	if (ret)
+		unpin_folios(folios, nr_folios);
+	return ret ? ret : nr_folios;
 }
 EXPORT_SYMBOL_GPL(memfd_pin_folios);
-- 
2.47.0