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Message-ID: <6ba088ae-4f84-6cd9-cbcc-bbc6b9547f04@redhat.com>
Date:   Mon, 28 Feb 2022 14:27:16 +0100
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Subject: Re: [RFC PATCH 1/7] mm/gup: introduce pin_user_page()
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To:     John Hubbard <jhubbard@nvidia.com>, Jens Axboe <axboe@kernel.dk>,
        Jan Kara <jack@suse.cz>, Christoph Hellwig <hch@infradead.org>,
        Dave Chinner <dchinner@redhat.com>,
        "Darrick J . Wong" <djwong@kernel.org>,
        Theodore Ts'o <tytso@mit.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Andrew Morton <akpm@linux-foundation.org>,
        Chaitanya Kulkarni <kch@nvidia.com>
Cc:     linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>
References: <20220225085025.3052894-1-jhubbard@nvidia.com>
 <20220225085025.3052894-2-jhubbard@nvidia.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20220225085025.3052894-2-jhubbard@nvidia.com>
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Precedence: bulk

On 25.02.22 09:50, John Hubbard wrote:
> pin_user_page() is an externally-usable version of try_grab_page(), but
> with semantics that match get_page(), so that it can act as a drop-in
> replacement for get_page(). Specifically, pin_user_page() has a void
> return type.
> 
> pin_user_page() elevates a page's refcount is using FOLL_PIN rules. This
> means that the caller must release the page via unpin_user_page().
> 
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
>  include/linux/mm.h |  1 +
>  mm/gup.c           | 34 ++++++++++++++++++++++++++++++++++
>  2 files changed, 35 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 929488a47181..bb51f5487aef 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1914,6 +1914,7 @@ long pin_user_pages_remote(struct mm_struct *mm,
>  long get_user_pages(unsigned long start, unsigned long nr_pages,
>  			    unsigned int gup_flags, struct page **pages,
>  			    struct vm_area_struct **vmas);
> +void pin_user_page(struct page *page);
>  long pin_user_pages(unsigned long start, unsigned long nr_pages,
>  		    unsigned int gup_flags, struct page **pages,
>  		    struct vm_area_struct **vmas);
> diff --git a/mm/gup.c b/mm/gup.c
> index 5c3f6ede17eb..44446241c3a9 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -3034,6 +3034,40 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
>  }
>  EXPORT_SYMBOL(pin_user_pages);
>  
> +/**
> + * pin_user_page() - apply a FOLL_PIN reference to a page ()
> + *
> + * @page: the page to be pinned.
> + *
> + * Similar to get_user_pages(), in that the page's refcount is elevated using
> + * FOLL_PIN rules.
> + *
> + * IMPORTANT: That means that the caller must release the page via
> + * unpin_user_page().
> + *
> + */
> +void pin_user_page(struct page *page)
> +{
> +	struct folio *folio = page_folio(page);
> +
> +	WARN_ON_ONCE(folio_ref_count(folio) <= 0);
> +
> +	/*
> +	 * Similar to try_grab_page(): be sure to *also*
> +	 * increment the normal page refcount field at least once,
> +	 * so that the page really is pinned.
> +	 */
> +	if (folio_test_large(folio)) {
> +		folio_ref_add(folio, 1);
> +		atomic_add(1, folio_pincount_ptr(folio));
> +	} else {
> +		folio_ref_add(folio, GUP_PIN_COUNTING_BIAS);
> +	}
> +
> +	node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, 1);
> +}
> +EXPORT_SYMBOL(pin_user_page);
> +
>  /*
>   * pin_user_pages_unlocked() is the FOLL_PIN variant of
>   * get_user_pages_unlocked(). Behavior is the same, except that this one sets

I assume that function will only get called on a page that has been
obtained by a previous pin_user_pages_fast(), correct?

-- 
Thanks,

David / dhildenb