On Thu, Mar 26, 2020 at 05:24:29AM -0700, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
>
> By moving PG_writeback down into the low bits of the page flags, we can
> use clear_bit_unlock_is_negative_byte() for writeback as well as the
> lock bit. wake_up_page() then has no more callers. Given the other
> code being executed between the clear and the test, this is not going
> to be as dramatic a win as it was for PageLocked, but symmetry between
> the two is nice and lets us remove some code.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
> include/linux/page-flags.h | 6 +++---
> mm/filemap.c | 19 ++++++-------------
> mm/page-writeback.c | 37 ++++++++++++++++++++-----------------
> 3 files changed, 29 insertions(+), 33 deletions(-)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 222f6f7b2bb3..96c7d220c8cf 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -103,13 +103,14 @@
> */
> enum pageflags {
> PG_locked, /* Page is locked. Don't touch. */
> + PG_writeback, /* Page is under writeback */
Do we need a comment why these need to be in the low bits?
