Re: [PATCH 2/6] block: add dio_w_*() wrappers for pin, unpin user pages

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 27 Aug 2022 01:36:03 -0700 John Hubbard <jhubbard@nvidia.com> wrote:

> Background: The Direct IO part of the block infrastructure is being
> changed to use pin_user_page*() and unpin_user_page*() calls, in place
> of a mix of get_user_pages_fast(), get_page(), and put_page(). These
> have to be changed over all at the same time, for block, bio, and all
> filesystems. However, most filesystems can be changed via iomap and core
> filesystem routines, so let's get that in place, and then continue on
> with converting the remaining filesystems (9P, CIFS) and anything else
> that feeds pages into bio that ultimately get released via
> bio_release_pages().
> 
> Add a new config parameter, CONFIG_BLK_USE_PIN_USER_PAGES_FOR_DIO, and
> dio_w_*() wrapper functions. The dio_w_ prefix was chosen for
> uniqueness, so as to ease a subsequent kernel-wide rename via
> search-and-replace. Together, these allow the developer to choose
> between these sets of routines, for Direct IO code paths:
> 
> a) pin_user_pages_fast()
>     pin_user_page()
>     unpin_user_page()
> 
> b) get_user_pages_fast()
>     get_page()
>     put_page()
> 
> CONFIG_BLK_USE_PIN_USER_PAGES_FOR_DIO is a temporary setting, and may
> be deleted once the conversion is complete. In the meantime, developers
> can enable this in order to try out each filesystem.
> 
> Please remember that these /proc/vmstat items (below) should normally
> contain the same values as each other, except during the middle of
> pin/unpin operations. As such, they can be helpful when monitoring test
> runs:
> 
>     nr_foll_pin_acquired
>     nr_foll_pin_released
> 
> ...
>
> +static inline void dio_w_unpin_user_pages(struct page **pages,
> +					  unsigned long npages)
> +{
> +	unsigned long i;
> +
> +	for (i = 0; i < npages; i++)
> +		put_page(pages[i]);
> +}

release_pages()?  Might be faster if many of the pages are page_count()==1.

(release_pages() was almost as simple as the above when I added it a
million years ago.  But then progress happened).