Re: [PATCH v8 08/26] mm/gup: allow FOLL_FORCE for get_user_pages_fast()

Jan Kara
2019-12-10T10:28:29Z

On Mon 09-12-19 14:53:26, 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@lst.de>
> Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>

Looks good to me. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> mm/gup.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index c0c56888e7cc..958ab0757389 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2414,7 +2414,8 @@ 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
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR