netdev - Re: [PATCH v8 08/26] mm/gup: allow FOLL_FORCE for get_user_pages

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Date:   Tue, 10 Dec 2019 11:28:18 +0100
From:   Jan Kara <jack@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, bpf@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, kvm@...r.kernel.org,
        linux-block@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-media@...r.kernel.org, linux-rdma@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Leon Romanovsky <leonro@...lanox.com>
Subject: Re: [PATCH v8 08/26] mm/gup: allow FOLL_FORCE for
 get_user_pages_fast()

On Mon 09-12-19 14:53:26, 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>
> Reviewed-by: Leon Romanovsky <leonro@...lanox.com>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>

Looks good to me. You can add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  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
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR