linux-kernel - [PATCHv7 2/3] mm/gup: fix omission of check on FOLL

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Date:   Tue, 17 Mar 2020 19:47:32 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     linux-mm@...ck.org
Cc:     Pingfan Liu <kernelfans@...il.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        John Hubbard <jhubbard@...dia.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Christoph Hellwig <hch@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Jason Gunthorpe <jgg@...pe.ca>,
        linux-kernel@...r.kernel.org
Subject: [PATCHv7 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

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@...il.com>
Cc: Ira Weiny <ira.weiny@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Mike Rapoport <rppt@...ux.ibm.com>
Cc: Dan Williams <dan.j.williams@...el.com>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: John Hubbard <jhubbard@...dia.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc: Christoph Hellwig <hch@...radead.org>
Cc: Shuah Khan <shuah@...nel.org>
Cc: Jason Gunthorpe <jgg@...pe.ca>
To: linux-mm@...ck.org
Cc: linux-kernel@...r.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