linux-kernel - Re: Folio discussion recap

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Date:   Mon, 13 Sep 2021 13:32:30 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: Folio discussion recap

On Sat 11-09-21 04:23:24, Kirill A. Shutemov wrote:
> On Fri, Sep 10, 2021 at 04:16:28PM -0400, Kent Overstreet wrote:
> > So we should listen to the MM people.
> 
> Count me here.
> 
> I think the problem with folio is that everybody wants to read in her/his
> hopes and dreams into it and gets disappointed when see their somewhat
> related problem doesn't get magically fixed with folio.
> 
> Folio started as a way to relief pain from dealing with compound pages.
> It provides an unified view on base pages and compound pages. That's it.
> 
> It is required ground work for wider adoption of compound pages in page
> cache. But it also will be useful for anon THP and hugetlb.
> 
> Based on adoption rate and resulting code, the new abstraction has nice
> downstream effects. It may be suitable for more than it was intended for
> initially. That's great.
> 
> But if it doesn't solve your problem... well, sorry...
> 
> The patchset makes a nice step forward and cuts back on mess I created on
> the way to huge-tmpfs.
> 
> I would be glad to see the patchset upstream.

I do agree here. While points that Johannes brought up are relevant
and worth thinking about I do also see a clear advantage of folio (or
whatever $name) is bringing. The compound page handling is just a mess
and source of practical problems and bugs.

This really requires some systematic approach to deal with it. The
proposed type system is definitely a good way to approach it. Johannes
is not happy about having the type still refer to page units but I
haven't seen an example where that leads to a worse or harder to
maintain code so far. The evolution is likely not going to stop at the
current type system but I haven't seen any specifics to prove it would
stand in the way. The existing code (fs or other subsystem interacting
with MM) is going to require quite a lot of changes to move away from
struct page notion but I do not see folios to add fundamental blocker
there.

All that being said, not only I see folios to be a step into the
right direction to address compound pages mess it is also a code that
already exists and gives some real advantages. I haven't heard anybody
subscribing to a different approach and providing an implementation in a
foreseeable future so I would rather go with this approach then dealing
with the existing code long term.
-- 
Michal Hocko
SUSE Labs