[PATCH v12 08/22] mm/gup: allow FOLL_FORCE for get_user_pages_fast()

From: John Hubbard
Date: Tue Jan 07 2020 - 17:47:38 EST

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@xxxxxx>
Reviewed-by: Leon Romanovsky <leonro@xxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
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_pages,
unsigned long addr, len, end;
int nr = 0, ret = 0;

- if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM)))
return -EINVAL;

start = untagged_addr(start) & PAGE_MASK;