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

From: John Hubbard
Date: Mon Dec 09 2019 - 14:54:40 EST

On 12/9/19 10:25 AM, Leon Romanovsky wrote:
> 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(-)
> Thanks,
> Reviewed-by: Leon Romanovsky <leonro@xxxxxxxxxxxx>

Hi Leon, thanks for the reviews, great timing! I'll add the tags to
the commits, which I'm just about to post as part of the larger
"pin user pages" patchset.

John Hubbard