Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Dan Williams]

On Mon, Dec 3, 2018 at 4:17 PM <john.hubbard@xxxxxxxxx> wrote:
>
> From: John Hubbard <jhubbard@xxxxxxxxxx>
>
> 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(), and also as a replacement
> for open-coded loops that release multiple pages.
> These may be used for subsequent performance improvements,
> via batching of pages to be released.
>
> This is the first step of fixing the problem described in [1]. The steps
> are:
>
> 1) (This patch): provide put_user_page*() routines, intended to be used
>    for releasing pages that were pinned via get_user_pages*().
>
> 2) Convert all of the call sites for get_user_pages*(), to
>    invoke put_user_page*(), instead of put_page(). This involves dozens of
>    call sites, and will take some time.
>
> 3) After (2) is complete, use get_user_pages*() and put_user_page*() to
>    implement tracking of these pages. This tracking will be separate from
>    the existing struct page refcounting.
>
> 4) Use the tracking and identification of these pages, to implement
>    special handling (especially in writeback paths) when the pages are
>    backed by a filesystem. Again, [1] provides details as to why that is
>    desirable.

I thought at Plumbers we talked about using a page bit to tag pages
that have had their reference count elevated by get_user_pages()? That
way there is no need to distinguish put_page() from put_user_page() it
just happens internally to put_page(). At the conference Matthew was
offering to free up a page bit for this purpose.

> [1] https://lwn.net/Articles/753027/ : "The Trouble with get_user_pages()"
>
> Reviewed-by: Jan Kara <jack@xxxxxxx>

Wish, you could have been there Jan. I'm missing why it's safe to
assume that a single put_user_page() is paired with a get_user_page()?