On Sun 31-05-20 22:26:32, John Hubbard wrote:
> There are four cases listed in pin_user_pages.rst. These are
> intended to help developers figure out whether to use
> get_user_pages*(), or pin_user_pages*(). However, the four cases
> do not cover all the situations. For example, drivers/vhost/vhost.c
> has a "pin, write to page, set page dirty, unpin" case.
>
> Add a fifth case, to help explain that there is a general pattern
> that requires pin_user_pages*() API calls.
>
> Cc: Vlastimil Babka <vba...@suse.cz>
> Cc: Jan Kara <j...@suse.cz>
> Cc: Jérôme Glisse <jgli...@redhat.com>
> Cc: Dave Chinner <da...@fromorbit.com>
> Cc: Jonathan Corbet <cor...@lwn.net>
> Cc: linux-...@vger.kernel.org
> Cc: linux-fsde...@vger.kernel.org
> Signed-off-by: John Hubbard <jhubb...@nvidia.com>
Looks good to me. You can add:
Reviewed-by: Jan Kara <j...@suse.cz>
Honza
> ---
> Documentation/core-api/pin_user_pages.rst | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> diff --git a/Documentation/core-api/pin_user_pages.rst
> b/Documentation/core-api/pin_user_pages.rst
> index 4675b04e8829..6068266dd303 100644
> --- a/Documentation/core-api/pin_user_pages.rst
> +++ b/Documentation/core-api/pin_user_pages.rst
> @@ -171,6 +171,24 @@ If only struct page data (as opposed to the actual
> memory contents that a page
> is tracking) is affected, then normal GUP calls are sufficient, and neither
> flag
> needs to be set.
>
> +CASE 5: Pinning in order to write to the data within the page
> +-------------------------------------------------------------
> +Even though neither DMA nor Direct IO is involved, just a simple case of
> "pin,
> +write to a page's data, unpin" can cause a problem. Case 5 may be considered
> a
> +superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
> +other words, if the code is neither Case 1 nor Case 2, it may still require
> +FOLL_PIN, for patterns like this:
> +
> +Correct (uses FOLL_PIN calls):
> + pin_user_pages()
> + write to the data within the pages
> + unpin_user_pages()
> +
> +INCORRECT (uses FOLL_GET calls):
> + get_user_pages()
> + write to the data within the pages
> + put_page()
> +
> page_maybe_dma_pinned(): the whole point of pinning
> ===================================================
>
> --
> 2.26.2
>
--
Jan Kara <j...@suse.com>
SUSE Labs, CR
