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Date:   Wed, 20 Oct 2021 23:51:52 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Kent Overstreet <kent.overstreet@gmail.com>,
        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>,
        Hugh Dickins <hughd@google.com>
Subject: Re: Folios for 5.15 request - Was: re: Folio discussion recap -
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On Wed, Oct 20, 2021 at 08:04:56PM +0200, David Hildenbrand wrote:
> real): assume we have to add a field for handling something about anon
> THP in the struct page (let's assume in the head page for simplicity).
> Where would we add it? To "struct folio" and expose it to all other
> folios that don't really need it because it's so special? To "struct
> page" where it actually doesn't belong after all the discussions? And if
> we would have to move that field it into a tail page, it would get even
> more "tricky".
> 
> Of course, we could let all special types inherit from "struct folio",
> which inherit from "struct page" ... but I am not convinced that we
> actually want that. After all, we're C programmers ;)
> 
> But enough with another side-discussion :)

FYI, with my block and direct I/O developer hat on I really, really
want to have the folio for both file and anon pages.  Because to make
the get_user_pages path a _lot_ more efficient it should store folios.
And to make that work I need them to work for file and anon pages
because for get_user_pages and related code they are treated exactly
the same.