[PATCH 5.4 148/152] mm/gup: allow FOLL_FORCE for get_user_pages_fast()

[Greg Kroah-Hartman]

From: John Hubbard <jhubbard@xxxxxxxxxx>

commit f4000fdf435b8301a11cf85237c561047f8c4c72 upstream.

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().

Link: http://lkml.kernel.org/r/20200107224558.2362728-9-jhubbard@xxxxxxxxxx
Fixes: 817be129e6f2 ("mm: validate get_user_pages_fast flags")
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
Reviewed-by: Leon Romanovsky <leonro@xxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
Cc: BjÃrn TÃpel <bjorn.topel@xxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Hans Verkuil <hverkuil-cisco@xxxxxxxxx>
Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Jerome Glisse <jglisse@xxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 mm/gup.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2401,7 +2401,8 @@ int get_user_pages_fast(unsigned long st
 	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;