Re: [PATCH v2 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"

[Jan Kara]

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 <vbabka@xxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>

Looks good to me. You can add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								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 <jack@xxxxxxxx>
SUSE Labs, CR