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From: John Hubbard <jhubbard@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>
CC: Al Viro <viro@zeniv.linux.org.uk>, Alex Williamson
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Subject: [PATCH v12 08/22] mm/gup: allow FOLL_FORCE for get_user_pages_fast()
Date: Tue, 7 Jan 2020 14:45:44 -0800
Message-ID: <20200107224558.2362728-9-jhubbard@nvidia.com>
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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>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jan Kara <jack@suse.cz>
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 b61bd5c469ae..a594bc708367 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2411,7 +2411,8 @@ int get_user_pages_fast(unsigned long start, int nr_p=
ages,
 	unsigned long addr, len, end;
 	int nr =3D 0, ret =3D 0;
=20
-	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM)))
+	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
+				       FOLL_FORCE)))
 		return -EINVAL;
=20
 	start =3D untagged_addr(start) & PAGE_MASK;
--=20
2.24.1