linux-kernel - Re: [PATCH 1/2] mm: introduce put_user

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Date:   Mon, 9 Jul 2018 09:53:57 -0600
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     john.hubbard@...il.com
Cc:     Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page(), placeholder version

On Mon, Jul 09, 2018 at 01:05:53AM -0700, john.hubbard@...il.com wrote:
> From: John Hubbard <jhubbard@...dia.com>
> 
> Introduces put_user_page(), which simply calls put_page().
> This provides a safe way to update all get_user_pages*() callers,
> so that they call put_user_page(), instead of put_page().
> 
> Also adds release_user_pages(), a drop-in replacement for
> release_pages(). This is intended to be easily grep-able,
> for later performance improvements, since release_user_pages
> is not batched like release_pages is, and is significantly
> slower.
> 
> Subsequent patches will add functionality to put_user_page().
> 
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
>  include/linux/mm.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a0fbb9ffe380..db4a211aad79 100644
> +++ b/include/linux/mm.h
> @@ -923,6 +923,20 @@ static inline void put_page(struct page *page)
>  		__put_page(page);
>  }
>  
> +/* Placeholder version, until all get_user_pages*() callers are updated. */
> +static inline void put_user_page(struct page *page)
> +{
> +	put_page(page);
> +}
> +
> +/* A drop-in replacement for release_pages(): */
> +static inline void release_user_pages(struct page **pages,
> +				      unsigned long npages)
> +{
> +	while (npages)
> +		put_user_page(pages[--npages]);
> +}
> +
>  #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
>  #define SECTION_IN_PAGE_FLAGS
>  #endif

Just as question: Do you think it is worthwhile to have a
release_user_page_dirtied() helper as well?

Ie to indicate that a pages that were grabbed under GUP FOLL_WRITE
were actually written too?

Keeps more of these unimportant details out of the drivers..

Jason