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

John Hubbard
2024-10-16T20:22:48Z

If a driver tries to call any of the pin_user_pages*(FOLL_LONGTERM)
family of functions, and requests "too many" pages, then the call will
erroneously leave pages pinned. This is visible in user space as an
actual memory leak.

Repro is trivial: just make enough pin_user_pages(FOLL_LONGTERM) calls
to exhaust memory.

The root cause of the problem is this sequence, within
__gup_longterm_locked():

__get_user_pages_locked()
rc = check_and_migrate_movable_pages()

...which gets retried in a loop. The loop error handling is incomplete,
clearly due to a somewhat unusual and complicated tri-state error API.
But anyway, if -ENOMEM, or in fact, any unexpected error is returned
from check_and_migrate_movable_pages(), then __gup_longterm_locked()
happily returns the error, while leaving the pages pinned.

In the failed case, which is an app that requests (via a device driver)
30720000000 bytes to be pinned, and then exits, I see this:

$ grep foll /proc/vmstat
nr_foll_pin_acquired 7502048
nr_foll_pin_released 2048

And after applying this patch, it returns to balanced pins:

$ grep foll /proc/vmstat
nr_foll_pin_acquired 7502048
nr_foll_pin_released 7502048

Fix this by unpinning the pages that __get_user_pages_locked() has
pinned, in such error cases.

Fixes: 24a95998e9ba ("mm/gup.c: simplify and fix check_and_migrate_movable_pages() return codes")
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Shigeru Yoshida <syoshida@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
mm/gup.c | 11 +++++++++++
1 file changed, 11 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index a82890b46a36..24acf53c8294 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2492,6 +2492,17 @@ static long __gup_longterm_locked(struct mm_struct *mm,

/* FOLL_LONGTERM implies FOLL_PIN */
rc = check_and_migrate_movable_pages(nr_pinned_pages, pages);
+
+ /*
+ * The __get_user_pages_locked() call happens before we know
+ * that whether it's possible to successfully complete the whole
+ * operation. To compensate for this, if we get an unexpected
+ * error (such as -ENOMEM) then we must unpin everything, before
+ * erroring out.
+ */
+ if (rc != -EAGAIN && rc != 0)
+ unpin_user_pages(pages, nr_pinned_pages);
+
} while (rc == -EAGAIN);
memalloc_pin_restore(flags);
return rc ? rc : nr_pinned_pages;
--
2.47.0