linux-kernel - Re: Folio discussion recap

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Date:   Fri, 17 Sep 2021 17:17:09 -0400
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Dave Chinner <david@...morbit.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.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>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: Folio discussion recap

On Fri, Sep 17, 2021 at 11:57:35PM +0300, Kirill A. Shutemov wrote:
> On Fri, Sep 17, 2021 at 12:31:36PM -0400, Johannes Weiner wrote:
> > I didn't suggest to change what the folio currently already is for the
> > page cache. I asked to keep anon pages out of it (and in the future
> > potentially other random stuff that is using compound pages).
> 
> It would mean that anon-THP cannot benefit from the work Willy did with
> folios. Anon-THP is the most active user of compound pages at the moment
> and it also suffers from the compound_head() plague. You ask to exclude
> anon-THP siting *possible* future benefits for pagecache.
> 
> Sorry, but this doesn't sound fair to me.

I'm less concerned with what's fair than figuring out what the consensus is so
we can move forward. I agree that anonymous THPs could benefit greatly from
conversion to folios - but looking at the code it doesn't look like much of that
has been done yet.

I understand you've had some input into the folio patches, so maybe you'd be
best able to answer while Matthew is away - would it be fair to say that, in the
interests of moving forward, anonymous pages could be split out for now? That
way the MM people gain time to come to their own consensus and we can still
unblock the FS work that's already been done on top of folios.