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Date:   Thu, 21 Oct 2021 09:21:17 +0200
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Subject: Re: Folios for 5.15 request - Was: re: Folio discussion recap -
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To:     Christoph Hellwig <hch@infradead.org>
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>,
        David Howells <dhowells@redhat.com>,
        Hugh Dickins <hughd@google.com>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 21.10.21 08:51, Christoph Hellwig wrote:
> 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.

Thanks, I can understand that. And IMHO that would be even possible with
split types; the function prototype will simply have to look a little
more fancy instead of replacing "struct page" by "struct folio". :)

-- 
Thanks,

David / dhildenb