On Sat, Mar 13, 2021 at 07:09:01PM -0800, Hugh Dickins wrote:

On Sat, 13 Mar 2021, Andrew Morton wrote:

quoted

On Fri,  5 Mar 2021 04:18:36 +0000 "Matthew Wilcox (Oracle)" [off-list ref] wrote:

quoted

Our type system does not currently distinguish between tail pages and
head or single pages.  This is a problem because we call compound_head()
multiple times (and the compiler cannot optimise it out), bloating the
kernel.  It also makes programming hard as it is often unclear whether
a function operates on an individual page, or an entire compound page.

This patch series introduces the struct folio, which is a type that
represents an entire compound page.  This initial set reduces the kernel
size by approximately 6kB, although its real purpose is adding
infrastructure to enable further use of the folio.

Geeze it's a lot of noise.  More things to remember and we'll forever
have a mismash of `page' and `folio' and code everywhere converting
from one to the other.  Ongoing addition of folio
accessors/manipulators to overlay the existing page
accessors/manipulators, etc.

It's unclear to me that it's all really worth it.  What feedback have
you seen from others?

My own feeling and feedback have been much like yours.

I don't get very excited by type safety at this level; and although
I protested back when all those compound_head()s got tucked into the
*PageFlag() functions, the text size increase was not very much, and
I never noticed any adverse performance reports.

To me, it's distraction, churn and friction, ongoing for years; but
that's just me, and I'm resigned to the possibility that it will go in.
Matthew is not alone in wanting to pursue it: let others speak.

I'm with Matthew on this. I would really want to drop the number of places
where we call compoud_head(). I hope we can get rid of the page flag
policy hack I made.

-- 
 Kirill A. Shutemov