linux-kernel - Re: [GIT PULL] Memory folios for v5.15

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Message-ID: <92cbfb8f-7418-15d5-c469-d7861e860589@rasmusvillemoes.dk>
Date:   Wed, 25 Aug 2021 11:01:04 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Christoph Hellwig <hch@...radead.org>,
        Theodore Ts'o <tytso@....edu>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] Memory folios for v5.15

On 25/08/2021 08.32, Christoph Hellwig wrote:
> On Tue, Aug 24, 2021 at 03:44:48PM -0400, Theodore Ts'o wrote:
>> The problem is whether we use struct head_page, or folio, or mempages,
>> we're going to be subsystem users' faces.  And people who are using it
>> every day will eventually get used to anything, whether it's "folio"
>> or "xmoqax", we sould give a thought to newcomers to Linux file system
>> code.  If they see things like "read_folio()", they are going to be
>> far more confused than "read_pages()" or "read_mempages()".
> 
> Are they?  It's not like page isn't some randomly made up term
> as well, just one that had a lot more time to spread.
> 
>> So if someone sees "kmem_cache_alloc()", they can probably make a
>> guess what it means, and it's memorable once they learn it.
>> Similarly, something like "head_page", or "mempages" is going to a bit
>> more obvious to a kernel newbie.  So if we can make a tiny gesture
>> towards comprehensibility, it would be good to do so while it's still
>> easier to change the name.
> 
> All this sounds really weird to me.  I doubt there is any name that
> nicely explains "structure used to manage arbitrary power of two
> units of memory in the kernel" very well.  So I agree with willy here,
> let's pick something short and not clumsy.  I initially found the folio
> name a little strange, but working with it I got used to it quickly.
> And all the other uggestions I've seen s far are significantly worse,
> especially all the odd compounds with page in it.
> 

A comment from the peanut gallery: I find the name folio completely
appropriate and easy to understand. Our vocabulary is already strongly
inspired by words used in the world of printed text: the smallest unit
of information is a char(acter) [ok, we usually call them bytes], a few
characters make up a word, there's a number of words to each (cache)
line, and a number of those is what makes up a page. So obviously a
folio is something consisting of a few pages.

Are the analogies perfect? Of course not. But they are actually quite
apt; words, lines and pages don't universally have one size, but they do
form a natural hierarchy describing how we organize information.

Splitting a word across lines can slow down the reader so should be
avoided... [sorry, couldn't resist].

Rasmus