linux-kernel - Re: [PATCH v3 06/12] mm/gup: require FOLL_GET for get_user_pages

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Message-ID: <20200203141811.GB18591@quack2.suse.cz>
Date:   Mon, 3 Feb 2020 15:18:11 +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>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 06/12] mm/gup: require FOLL_GET for
 get_user_pages_fast()

On Fri 31-01-20 19:40:23, John Hubbard wrote:
> Internal to mm/gup.c, require that get_user_pages_fast()
> and __get_user_pages_fast() identify themselves, by setting
> FOLL_GET. This is required in order to be able to make decisions
> based on "FOLL_PIN, or FOLL_GET, or both or neither are set", in
> upcoming patches.
> 
> Signed-off-by: John Hubbard <jhubbard@...dia.com>

Looks good. You can add:

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

								Honza

> ---
>  mm/gup.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 83473c2165f4..e899d2e6398c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2390,6 +2390,14 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>  	unsigned long len, end;
>  	unsigned long flags;
>  	int nr = 0;
> +	/*
> +	 * Internally (within mm/gup.c), gup fast variants must set FOLL_GET,
> +	 * because gup fast is always a "pin with a +1 page refcount" request.
> +	 */
> +	unsigned int gup_flags = FOLL_GET;
> +
> +	if (write)
> +		gup_flags |= FOLL_WRITE;
>  
>  	start = untagged_addr(start) & PAGE_MASK;
>  	len = (unsigned long) nr_pages << PAGE_SHIFT;
> @@ -2415,7 +2423,7 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>  	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
>  	    gup_fast_permitted(start, end)) {
>  		local_irq_save(flags);
> -		gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr);
> +		gup_pgd_range(start, end, gup_flags, pages, &nr);
>  		local_irq_restore(flags);
>  	}
>  
> @@ -2454,7 +2462,7 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
>  	int nr = 0, ret = 0;
>  
>  	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
> -				       FOLL_FORCE | FOLL_PIN)))
> +				       FOLL_FORCE | FOLL_PIN | FOLL_GET)))
>  		return -EINVAL;
>  
>  	start = untagged_addr(start) & PAGE_MASK;
> @@ -2521,6 +2529,13 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
>  	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
>  		return -EINVAL;
>  
> +	/*
> +	 * The caller may or may not have explicitly set FOLL_GET; either way is
> +	 * OK. However, internally (within mm/gup.c), gup fast variants must set
> +	 * FOLL_GET, because gup fast is always a "pin with a +1 page refcount"
> +	 * request.
> +	 */
> +	gup_flags |= FOLL_GET;
>  	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
>  }
>  EXPORT_SYMBOL_GPL(get_user_pages_fast);
> -- 
> 2.25.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR