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

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Fri, May 29, 2020 at 04:43:08PM -0700, John Hubbard wrote:
> +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,
> +access 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()
> +    access the data within the pages
> +    set_page_dirty_lock()
> +    unpin_user_pages()
> +
> +INCORRECT (uses FOLL_GET calls):
> +    get_user_pages()
> +    access the data within the pages
> +    set_page_dirty_lock()
> +    put_page()

Why does this case need to pin?  Why can't it just do ...

	get_user_pages()
	lock_page(page);
	... modify the data ...
	set_page_dirty(page);
	unlock_page(page);