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Date:   Wed, 22 Sep 2021 17:26:23 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Kent Overstreet <kent.overstreet@gmail.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        David Howells <dhowells@redhat.com>
Subject: Re: Folios for 5.15 request - Was: re: Folio discussion recap -
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On Wed, Sep 22, 2021 at 11:46:04AM -0400, Kent Overstreet wrote:
> On Wed, Sep 22, 2021 at 11:08:58AM -0400, Johannes Weiner wrote:
> > On Tue, Sep 21, 2021 at 05:22:54PM -0400, Kent Overstreet wrote:
> > >  - it's become apparent that there haven't been any real objections to the code
> > >    that was queued up for 5.15. There _are_ very real discussions and points of
> > >    contention still to be decided and resolved for the work beyond file backed
> > >    pages, but those discussions were what derailed the more modest, and more
> > >    badly needed, work that affects everyone in filesystem land
> > 
> > Unfortunately, I think this is a result of me wanting to discuss a way
> > forward rather than a way back.
> > 
> > To clarify: I do very much object to the code as currently queued up,
> > and not just to a vague future direction.
> > 
> > The patches add and convert a lot of complicated code to provision for
> > a future we do not agree on. The indirections it adds, and the hybrid
> > state it leaves the tree in, make it directly more difficult to work
> > with and understand the MM code base. Stuff that isn't needed for
> > exposing folios to the filesystems.
> > 
> > As Willy has repeatedly expressed a take-it-or-leave-it attitude in
> > response to my feedback, I'm not excited about merging this now and
> > potentially leaving quite a bit of cleanup work to others if the
> > downstream discussion don't go to his liking.

We're at a take-it-or-leave-it point for this pull request.  The time
for discussion was *MONTHS* ago.

> > Here is the roughly annotated pull request:
> 
> Thanks for breaking this out, Johannes.
> 
> So: mm/filemap.c and mm/page-writeback.c - I disagree about folios not really
> being needed there. Those files really belong more in fs/ than mm/, and the code
> in those files needs folios the most - especially filemap.c, a lot of those
> algorithms have to change from block based to extent based, making the analogy
> with filesystems.
> 
> I think it makes sense to drop the mm/lru stuff, as well as the mm/memcg,
> mm/migrate and mm/workingset and mm/swap stuff that you object to - that is, the
> code paths that are for both file + anonymous pages, unless Matthew has
> technical reasons why that would break the rest of the patch set.

Conceptually, it breaks the patch set.  Anywhere that we convert back
from a folio to a page, the guarantee of folios is weakened (and
possibly violated).  I don't think it makes sense from a practical point
of view either; it's re-adding compound_head() calls that just don't
need to be there.

> That discussion can still happen... and there's still the potential to get a lot
> more done if we're breaking open struct page and coming up with new types. I got
> Matthew on board with what you wanted, re: using the slab allocator for larger
> allocations

Wait, no, you didn't.  I think it's a terrible idea.  It's just completely
orthogonal to this patch set, so I don't want to talk about it.