linux-kernel - Re: [PATCH v2 1/2] mm/gup: allow FOLL_FORCE for get_user_pages

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Message-ID: <7ceb9a54-6e2f-6d1f-70ba-f6ad5ba0b37f@nvidia.com>
Date:   Mon, 9 Dec 2019 11:54:36 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Leon Romanovsky <leon@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christoph Hellwig <hch@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>, <linux-rdma@...r.kernel.org>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 1/2] mm/gup: allow FOLL_FORCE for get_user_pages_fast()

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@....de>
>> Signed-off-by: John Hubbard <jhubbard@...dia.com>
>> ---
>>  mm/gup.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
> 
> Thanks,
> Reviewed-by: Leon Romanovsky <leonro@...lanox.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