Re: Folios: Can we resolve this please?

From: David Hildenbrand
Date: Wed Sep 01 2021 - 06:18:16 EST

On 01.09.21 00:15, David Howells wrote:
Hi Linus, Andrew, Johannes,

Can we come to a quick resolution on folios? I'd really like this to be
solved in this merge window if at all possible as I (and others) have stuff
that will depend on and will conflict with Willy's folio work. It would be
great to get this sorted one way or another.

As I see it, there are three issues, I think, and I think they kind of go like

(1) Johannes wants to get away from pages being used as the unit of memory
currency and thinks that folios aren't helpful in this regard[1]. There
seems to be some disagreement about where this is heading.

(2) Linus isn't entirely keen on Willy's approach[2], with a bottom up
approach hiding the page objects behind a new type from the pov of the
filesystem, but would rather see the page struct stay the main API type
and the changes be hidden transparently inside of that.

I think from what Linus said, he may be in favour (if that's not too
strong a word) of using a new type to make sure we don't miss the
necessary changes[3].

(3) Linus isn't in favour of the name 'folio' for the new type[2]. Various
names have been bandied around and Linus seems okay with "pageset"[4],
though it's already in minor(-ish) use[5][6]. Willy has an alternate
patchset with "folio" changed to "pageset"[7].

With regard to (1), I think the folio concept could be used in future to hide
at least some of the paginess from filesystems.

With regard to (2), I think a top-down approach won't work until and unless we
wrap all accesses to struct page by filesystems (and device drivers) in
wrapper functions - we need to stop filesystems fiddling with page internals
because what page internals may mean may change.

With regard to (3), I'm personally fine with the name "folio", as are other
people[8][9][10][11], but I could also live with a conversion to "pageset".

Is it possible to take the folios patchset as-is and just live with the name,
or just take Willy's rename-job (although it hasn't had linux-next soak time
yet)? Or is the approach fundamentally flawed and in need of redoing?

Whatever we do, it would be great to get it out of -next one way (merge) or the other (drop) ASAP, as it's a lot of code churn, affecting various subsystems.

But merging it in a (for some people) suboptimal state just to get it out of -next might not necessarily be what we want.


David / dhildenb