linux-kernel - Re: [RFC PATCH 00/12] remove is_swap

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Message-ID: <10862498-3348-4cf8-a00d-355d12100443@lucifer.local>
Date: Fri, 24 Oct 2025 21:14:10 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>, Zi Yan <ziy@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Lance Yang <lance.yang@...ux.dev>,
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        kvm@...r.kernel.org, linux-s390@...r.kernel.org,
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        linux-mm@...ck.org
Subject: Re: [RFC PATCH 00/12] remove is_swap_[pte, pmd]() + non-swap
 confusion

On Fri, Oct 24, 2025 at 08:05:33PM +0000, Yosry Ahmed wrote:
> On Fri, Oct 24, 2025 at 08:41:16AM +0100, Lorenzo Stoakes wrote:
> > There's an established convention in the kernel that we treat leaf page
> > tables (so far at the PTE, PMD level) as containing 'swap entries' should
> > they be neither empty (i.e. p**_none() evaluating true) nor present
> > (i.e. p**_present() evaluating true).
> >
> > However, at the same time we also have helper predicates - is_swap_pte(),
> > is_swap_pmd() - which are inconsistently used.
> >
> > This is problematic, as it is logical to assume that should somebody wish
> > to operate upon a page table swap entry they should first check to see if
> > it is in fact one.
> >
> > It also implies that perhaps, in future, we might introduce a non-present,
> > none page table entry that is not a swap entry.
> >
> > This series resolves this issue by systematically eliminating all use of
> > the is_swap_pte() and is swap_pmd() predicates so we retain only the
> > convention that should a leaf page table entry be neither none nor present
> > it is a swap entry.
> >
> > We also have the further issue that 'swap entry' is unfortunately a really
> > rather overloaded term and in fact refers to both entries for swap and for
> > other information such as migration entries, page table markers, and device
> > private entries.
> >
> > We therefore have the rather 'unique' concept of a 'non-swap' swap entry.
> >
> > This is deeply confusing, so this series goes further and eliminates the
> > non_swap_entry() predicate, replacing it with is_non_present_entry() - with
> > an eye to a new convention of referring to these non-swap 'swap entries' as
> > non-present.
>
> I just wanted to say THANK YOU for doing this. It is indeed a very
> annoying and confusing convention, and I wanted to do something about it
> in the past but never got around to it..

:) that's very kind of you to say thanks! I was motivated by pure irritation at
this situation after David and I discussed it extensively.

I was initially thinking we should consistently _use_ the predicate and was
arguing for it on review, but David pointed out the convention is quite the
opposite and it became apparent to me th the predicates were the issue.

Of course I have encountered this non-swap swap entry madness as all of us
in mm have, but fixing that ends up being a natural extension to resolving
the is_swap_xxx() stuff and a big relief to deal with also.

There's more work to be done but this addresses some of the more proximate
issues! :)

Cheers, Lorenzo