linux-kernel - Re: [PATCH v2 2/3] mm: introduce put_user

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Message-ID: <20181005151726.GA20776@ziepe.ca>
Date:   Fri, 5 Oct 2018 09:17:26 -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>, 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>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jerome Glisse <jglisse@...hat.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v2 2/3] mm: introduce put_user_page[s](), placeholder
 versions

On Thu, Oct 04, 2018 at 09:02:24PM -0700, john.hubbard@...il.com wrote:
> From: John Hubbard <jhubbard@...dia.com>
> 
> Introduces put_user_page(), which simply calls put_page().
> This provides a way to update all get_user_pages*() callers,
> so that they call put_user_page(), instead of put_page().
> 
> Also introduces put_user_pages(), and a few dirty/locked variations,
> as a replacement for release_pages(), for the same reasons.
> These may be used for subsequent performance improvements,
> via batching of pages to be released.
> 
> This prepares for eventually fixing the problem described
> in [1], and is following a plan listed in [2], [3], [4].
> 
> [1] https://lwn.net/Articles/753027/ : "The Trouble with get_user_pages()"
> 
> [2] https://lkml.kernel.org/r/20180709080554.21931-1-jhubbard@nvidia.com
>     Proposed steps for fixing get_user_pages() + DMA problems.
> 
> [3]https://lkml.kernel.org/r/20180710082100.mkdwngdv5kkrcz6n@quack2.suse.cz
>     Bounce buffers (otherwise [2] is not really viable).
> 
> [4] https://lkml.kernel.org/r/20181003162115.GG24030@quack2.suse.cz
>     Follow-up discussions.
> 
> CC: Matthew Wilcox <willy@...radead.org>
> CC: Michal Hocko <mhocko@...nel.org>
> CC: Christopher Lameter <cl@...ux.com>
> CC: Jason Gunthorpe <jgg@...pe.ca>
> CC: Dan Williams <dan.j.williams@...el.com>
> CC: Jan Kara <jack@...e.cz>
> CC: Al Viro <viro@...iv.linux.org.uk>
> CC: Jerome Glisse <jglisse@...hat.com>
> CC: Christoph Hellwig <hch@...radead.org>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
>  include/linux/mm.h | 42 ++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 40 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a61ebe8ad4ca..1a9aae7c659f 100644
> +++ b/include/linux/mm.h
> @@ -137,6 +137,8 @@ extern int overcommit_ratio_handler(struct ctl_table *, int, void __user *,
>  				    size_t *, loff_t *);
>  extern int overcommit_kbytes_handler(struct ctl_table *, int, void __user *,
>  				    size_t *, loff_t *);
> +int set_page_dirty(struct page *page);
> +int set_page_dirty_lock(struct page *page);
>  
>  #define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + (n))
>  
> @@ -943,6 +945,44 @@ 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);
> +}
> +
> +/* For get_user_pages*()-pinned pages, use these variants instead of
> + * release_pages():
> + */
> +static inline void put_user_pages_dirty(struct page **pages,
> +					unsigned long npages)
> +{
> +	while (npages) {
> +		set_page_dirty(pages[npages]);
> +		put_user_page(pages[npages]);
> +		--npages;
> +	}
> +}

Shouldn't these do the !PageDirty(page) thing?

Jason