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Date:   Mon, 15 Mar 2021 14:55:01 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v4 00/25] Page folios
Message-ID: <20210315115501.7rmzaan2hxsqowgq@box>
References: <20210305041901.2396498-1-willy@infradead.org>
 <20210313123658.ad2dcf79a113a8619c19c33b@linux-foundation.org>
 <alpine.LSU.2.11.2103131842590.14125@eggly.anvils>
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On Sat, Mar 13, 2021 at 07:09:01PM -0800, Hugh Dickins wrote:
> On Sat, 13 Mar 2021, Andrew Morton wrote:
> > On Fri,  5 Mar 2021 04:18:36 +0000 "Matthew Wilcox (Oracle)" <willy@infradead.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.

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