linux-kernel - Re: [PATCH v4 00/25] Page folios

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Message-ID: <alpine.LSU.2.11.2103131842590.14125@eggly.anvils>
Date:   Sat, 13 Mar 2021 19:09:01 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4 00/25] Page folios

On Sat, 13 Mar 2021, Andrew Morton wrote:
> On Fri,  5 Mar 2021 04:18:36 +0000 "Matthew Wilcox (Oracle)" <willy@...radead.org> wrote:
> 
> > 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.

Hugh