linux-kernel - Re: [PATCH 1/6] mm/gup: introduce pin_user

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Message-ID: <10a9d33a-58a3-10b3-690b-53100d4e5440@redhat.com>
Date:   Mon, 29 Aug 2022 14:07:52 +0200
From:   David Hildenbrand <david@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>, Jan Kara <jack@...e.cz>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] mm/gup: introduce pin_user_page()

On 27.08.22 10:36, 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 using FOLL_PIN rules. This
> means that the caller must release the page via unpin_user_page().
> 
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  include/linux/mm.h |  1 +
>  mm/gup.c           | 33 +++++++++++++++++++++++++++++++++
>  2 files changed, 34 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 982f2607180b..85a105157334 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1869,6 +1869,7 @@ 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 5abdaf487460..245ccb41ed8c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -3213,6 +3213,39 @@ 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.
> + *
> + * This is similar to get_user_pages(), except that the page's refcount is
> + * elevated using FOLL_PIN, instead of FOLL_GET.
> + *
> + * IMPORTANT: 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);
> +

We should warn if the page is anon and !exclusive.

I assume the intend is to use pin_user_page() only to duplicate pins, right?


-- 
Thanks,

David / dhildenb