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    Subject[PATCH v8 08/26] mm/gup: allow FOLL_FORCE for get_user_pages_fast()
    Date
    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>
    ---
    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
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