linux-kernel - Re: [PATCH RFC v2 0/3] mm: Properly document tail pages for a folio

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Message-ID: <ZNvidA8/FkfHk/YU@x1n>
Date:   Tue, 15 Aug 2023 16:39:16 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Ryan Roberts <ryan.roberts@....com>,
        Yang Shi <shy828301@...il.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH RFC v2 0/3] mm: Properly document tail pages for a folio

On Tue, Aug 15, 2023 at 09:16:47PM +0100, Matthew Wilcox wrote:
> No, sometimes there are things which shouldn't be documented because they
> don't matter, and when changing code sometimes we forget to change the
> documentation, and then people read the documentation which is different
> from the code, and they get confused.
> 
> It matters that the various 'private' members line up.  It matters
> that folio->index matches page->index.  It does not matter what
> offset _entire_mapcount is at.  That can be moved around freely and no
> documentation needs to be changed.
> 
> I don't want you to use FOLIO_MATCH to make any unnecessary assertions.
> The only assertion missing is for _private_1 and _private_2a, and that's
> why I wrote a patch to add them.

I didn't mean to add assertions for _entire_mapcount (I don't even know
how..), but _mapcount and _refcount to clamp the fields, then all fields
can be clear, just like head/flags clamping the start of fields.

One thing I can understand that you'd like to avoid these "offset" things
is perhaps because you keep that in mind to, one day, have mmdesc replacing
folio so folio doesn't need to match struct page at all some day,
ideally. The order of fields, size of fields, etc. none of them should
matter, when it comes, and we should go toward that.  However my argument
would be that, before that day comes IMHO we need some good documentation
for us to know how the fields look like now, why they worked, and how to
reuse new fields.. when that comes, we can just safely remove these
documentations.

It's just that these 'offset's still matter and matters a lot now, imho,
but it's very confusing when read without some help.

Let me try one more time to see how you think about it on an rfcv3.  If
that still doesn't get any form of ack from you, I'll put this aside.

-- 
Peter Xu