[PATCHv8 2/2] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Pingfan Liu <kernelfans@gmail.com>]

FOLL_LONGTERM is a special case of FOLL_PIN. It suggests a pin which is
going to be given to hardware and can't move. It would truncate CMA
permanently and should be excluded.

In gup slow path, where
__gup_longterm_locked->check_and_migrate_cma_pages() handles FOLL_LONGTERM,
but in fast path, there lacks such a check, which means a possible leak of
CMA page to longterm pinned.

Place a check in try_grab_compound_head() in the fast path to fix the leak,
and if FOLL_LONGTERM happens on CMA, it will fall back to slow path to
migrate the page.

Some note about the check:
Huge page's subpages have the same migrate type due to either
allocation from a free_list[] or alloc_contig_range() with param
MIGRATE_MOVABLE. So it is enough to check on a single subpage
by is_migrate_cma_page(subpage)

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
To: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
 mm/gup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index 02a95b1..3fe75c4 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -89,6 +89,14 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
 		int orig_refs = refs;

 		/*
+		 * Can't do FOLL_LONGTERM + FOLL_PIN with CMA in the gup fast
+		 * path, so fail and let the caller fall back to the slow path.
+		 */
+		if (unlikely(flags & FOLL_LONGTERM) &&
+				is_migrate_cma_page(page))
+			return NULL;
+
+		/*
 		 * When pinning a compound page of order > 1 (which is what
 		 * hpage_pincount_available() checks for), use an exact count to
 		 * track it, via hpage_pincount_add/_sub().
--
2.7.5