[PATCHv7 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

Pingfan Liu
2020-03-17T11:49:50Z

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, 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>
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
---
v6 -> v7: fix coding style issue
mm/gup.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index 9df77b1..0a536d7 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -89,6 +89,15 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
int orig_refs = refs;

/*
+ * 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 subpage.
+ */
+ 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