Re: [PATCH v2 1/2] mm/gup: allow FOLL_FORCE for get_user_pages_fast()

From: Leon Romanovsky
Date: Mon Dec 09 2019 - 13:25:43 EST

On Wed, Dec 04, 2019 at 01:36:02PM -0800, John Hubbard wrote:
> 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>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
> mm/gup.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)

Reviewed-by: Leon Romanovsky <leonro@xxxxxxxxxxxx>