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Date:   Thu, 21 Oct 2021 15:00:08 +0200
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To:     Christoph Hellwig <hch@infradead.org>
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        "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
Subject: Re: Folios for 5.15 request - Was: re: Folio discussion recap -
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On 21.10.21 14:38, Christoph Hellwig wrote:
> On Thu, Oct 21, 2021 at 02:35:32PM +0200, David Hildenbrand wrote:
>> My opinion after all the discussions: use a dedicate type with a clear
>> name to solve the immediate filemap API issue. Leave the remainder alone
>> for now. Less code to touch, less subsystems to involve (well, still a
>> lot), less people to upset, less discussions to have, faster review,
>> faster upstream, faster progress. A small but reasonable step.
> 
> I don't get it.  I mean I'm not the MM expert, I've only been touching
> most areas of it occasionally for the last 20 years, but anon and file
> pages have way more in common both in terms of use cases and

You most certainly have way more MM expertise than me ;) I'm just a
random MM developer, so everybody can feel free to just ignore what I'm
saying here. I didn't NACK anything, I just consider a lot of things
that Johannes raised reasonable.

> implementation than what is different (unlike some of the other (ab)uses
> of struct page).  What is the point of splitting it now when there are
> tons of use cases where they are used absolutely interchangable both
> in consumers of the API and the implementation?
I guess in an ideal world, we'd have multiple abstractions. We could
clearly express for a function what type it expects. We'd have a type
for something passed on the filemap API. We'd have a type for anon THP
(or even just an anon page). We'd have a type that abstracts both.

With that in mind, and not planning with what we'll actually end up
with, to me it makes perfect sense to teach the filemap API to consume
the expected type first. And I am not convinced that the folio as is
("not a tail page") is the right abstraction we actually want to pass
around in places where we expect either anon or file pages -- or only
anon pages or only file pages.

Again, my 2 cents.

-- 
Thanks,

David / dhildenb