[PATCH 1/2] mm/gup: allow FOLL_FORCE for get_user_pages_fast()

From: John Hubbard <jhubbard@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>,
	Jason Gunthorpe <jgg@ziepe.ca>, Leon Romanovsky <leon@kernel.org>,
	Christoph Hellwig <hch@infradead.org>
Cc: Ira Weiny <ira.weiny@intel.com>, <linux-rdma@vger.kernel.org>,
	<linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
	John Hubbard <jhubbard@nvidia.com>,
	Christoph Hellwig <hch@lst.de>
Subject: [PATCH 1/2] mm/gup: allow FOLL_FORCE for get_user_pages_fast()
Date: Sun, 24 Nov 2019 16:37:14 -0800	[thread overview]
Message-ID: <20191125003715.516290-2-jhubbard@nvidia.com> (raw)
In-Reply-To: <20191125003715.516290-1-jhubbard@nvidia.com>

Commit 817be129e6f2 ("mm: validate get_user_pages_fast flags") allowed
only FOLL_WRITE and FOLL_LONGTERM to be passed to get_user_pages_fast().
This, combined with the fact that get_user_pages_fast() falls back to
"slow gup", which *does* accept FOLL_FORCE, leads to an odd situation:
if you need FOLL_FORCE, you cannot call get_user_pages_fast().

There does not appear to be any reason for filtering out FOLL_FORCE.
There is nothing in the _fast() implementation that requires that we
avoid writing to the pages. So it appears to have been an oversight.

Fix by allowing FOLL_FORCE to be set for get_user_pages_fast().

Fixes: 817be129e6f2 ("mm: validate get_user_pages_fast flags")
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index 8f236a335ae9..745b4036cdfd 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2401,7 +2401,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
 	unsigned long addr, len, end;
 	int nr = 0, ret = 0;
 
-	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM)))
+	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
+				       FOLL_FORCE)))
 		return -EINVAL;
 
 	start = untagged_addr(start) & PAGE_MASK;
-- 
2.24.0

