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

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Message-ID: <20191209182536.GC67461@unreal>
Date:   Mon, 9 Dec 2019 20:25:36 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
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 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>