Re: [LSF/MM TOPIC] get_user_pages() pins in file mappings

[John Hubbard <jhubbard@nvidia.com>]

On 1/24/19 1:04 AM, Jan Kara wrote:

> In particular we hope to have reasonably robust mechanism of identifying
> pages pinned by GUP (patches will be posted soon) - I'd like to run that by
> MM folks (unless discussion happens on mailing lists before LSF/MM). We
> also have ideas how filesystems should react to pinned page in their
> writepages methods - there will be some changes needed in some filesystems
> to bounce the page if they need stable page contents. So I'd like to
> explain why we chose to do bouncing to fs people (i.e., why we cannot just
> wait, skip the page, do something else etc.) to save us from the same
> discussion with each fs separately and also hash out what the API for
> filesystems to do this should look like. Finally we plan to keep pinned
> page permanently dirty - again something I'd like to explain why we do this
> and gather input from other people.

Hi Jan,

Say, I was just talking through this point with someone on our driver team, 
and suddenly realized that I'm now slightly confused on one point. If we end
up keeping the gup-pinned pages effectively permanently dirty while pinned,
then maybe the call sites no longer need to specify "dirty" (or not) when
they call put_user_page*()?

In other words, the RFC [1] has this API:

    void put_user_page(struct page *page);
    void put_user_pages_dirty(struct page **pages, unsigned long npages);
    void put_user_pages_dirty_lock(struct page **pages, unsigned long npages);
    void put_user_pages(struct page **pages, unsigned long npages);

But maybe we only really need this:

    void put_user_page(struct page *page);
    void put_user_pages(struct page **pages, unsigned long npages);

?

[1] https://lkml.kernel.org/r/20190204052135.25784-1-jhubbard@nvidia.com

thanks,
-- 
John Hubbard
NVIDIA