linux-kernel - Re: Folios for 5.15 request - Was: re: Folio discussion recap -

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Date:   Fri, 22 Oct 2021 22:02:33 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        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>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: Folios for 5.15 request - Was: re: Folio discussion recap -

On Sat, Oct 23, 2021 at 03:22:35AM +0100, Matthew Wilcox wrote:
> You can see folios as a first step to disentangling some of the users
> of struct page.  It certainly won't be the last step.  But I'd really
> like to stop having theoretical discussions of memory types and get on
> with writing code.

Agreed.  I think folios are really important to sort out the mess
around compound pages ASAP.

I'm a lot more lukewarm on the other splits.  Yes, struct page is a
mess, but I'm not sure creating gazillions of new types solve that
mess.  Getting rid of a bunch of the crazy optimizations that abuse
struct page fields might a better first step - or rather after the
first step of folios which fix real bugs in compount handling and do
enable sane handling of compound pages in the page cache.

> If that means we modify the fs APIs again in twelve
> months to replace folios with file_mem, well, I'm OK with that.

I suspect we won't even need that so quickly if at all, but I'd rather
have a little more churn rather than blocking this important work
forever.