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

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Date:   Tue, 24 Aug 2021 14:04:56 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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 Mon, Aug 23, 2021 at 03:06:08PM -0700, Linus Torvalds wrote:
> Yeah, honestly, I would have preferred to see this done the exact
> reverse way: make the rule be that "struct page" is always a head
> page, and anything that isn't a head page would be called something
> else.
> 
> Because, as you say, head pages are the norm. And "folio" may be a
> clever term, but it's not very natural. Certainly not at all as
> intuitive or common as "page" as a name in the industry.

Actually, I think this is an advantage for folios.  Maybe not for the
core MM which has always been _fairly_ careful to deal with compound
pages properly.  But for filesystem people, device drivers, etc, when
people see a struct page, they think it's PAGE_SIZE bytes in size.
And they're usually right, which is what makes things like THP so prone
to "Oops, we missed a spot" bugs.  By contrast, if you see something
which takes a struct folio and then works on PAGE_SIZE bytes, that's a
sign there's something funny going on.  There are a few of those still;
for example kmap() can only map PAGE_SIZE bytes at a time.