linux-kernel - Re: Folio discussion recap

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Date:   Fri, 17 Sep 2021 23:57:35 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Dave Chinner <david@...morbit.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        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 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.

We already had similar experiment with PAGE_CACHE_SIZE. It was introduced
with hope to have PAGE_CACHE_SIZE != PAGE_SIZE one day. It never happened
and only caused confusion on the border between pagecache-specific code
and generic code that handled both file and anon pages.

If you want to limit usage of the new type to pagecache, the burden on you
to prove that it is useful and not just a dead weight.

-- 
 Kirill A. Shutemov