Date:   Tue, 31 Aug 2021 23:15:47 +0100
From:   David Howells <dhowells@...hat.com>
To:     torvalds@...ux-foundation.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>
cc:     dhowells@...hat.com, Matthew Wilcox <willy@...radead.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Christoph Hellwig <hch@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Vlastimil Babka <vbabka@...e.cz>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Jeff Layton <jlayton@...nel.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Folios: Can we resolve this please?

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
this:

 (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?

Thanks,
David

Link: https://lore.kernel.org/r/YSQSkSOWtJCE4g8p@cmpxchg.org/ [1]
Link: https://lore.kernel.org/r/CAHk-=wjD8i2zJVQ9SfF2t=_0Fkgy-i5Z=mQjCw36AHvbBTGXyg@mail.gmail.com/ [2]
Link: https://lore.kernel.org/r/CAHk-=wgkA=RKJ-vke0EoOUK19Hv1f=47Da6pWAWQZPhjKD6WOg@mail.gmail.com/ [3]
Link: https://lore.kernel.org/r/CAHk-=wiZ=wwa4oAA0y=Kztafgp0n+BDTEV6ybLoH2nvLBeJBLA@mail.gmail.com/ [4]
Link: https://lore.kernel.org/r/CAHk-=whd8ugrzMS-3bupkPQz9VS+dWHPpsVssrDfuFgfff+n5A@mail.gmail.com/ [5]
Link: https://lore.kernel.org/r/CAHk-=wgwRW1_o6iBOxtSE+vm7uiSr98wkTLbCze9-7wW0ZhOLQ@mail.gmail.com/ [6]
Link: https://lore.kernel.org/r/YSmtjVTqR9%2F4W1aq@casper.infradead.org/ [7]
Link: https://lore.kernel.org/r/YSXkDFNkgAhQGB0E@infradead.org/ [8]
Link: https://lore.kernel.org/r/92cbfb8f-7418-15d5-c469-d7861e860589@rasmusvillemoes.dk/ [9]
Link: https://lore.kernel.org/r/cf30c0e8d1eecf08b2651c5984ff09539e2266f9.camel@kernel.org/ [10]
Link: https://lore.kernel.org/r/20210826005914.GG12597@magnolia/ [11]

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