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Subject: Re: [PATCH v2 1/2] mm/gup: allow FOLL_FORCE for get_user_pages_fast()
To:     Leon Romanovsky <leon@kernel.org>
CC:     Andrew Morton <akpm@linux-foundation.org>,
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References: <20191204213603.464373-1-jhubbard@nvidia.com>
 <20191204213603.464373-2-jhubbard@nvidia.com> <20191209182536.GC67461@unreal>
From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Mon, 9 Dec 2019 11:54:36 -0800
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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@lst.de>
>> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
>> ---
>>  mm/gup.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
> 
> Thanks,
> Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
> 

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.


thanks,
-- 
John Hubbard
NVIDIA