linux-kernel - Re: [PATCH RFC 00/14] mm: vm_normal_page*() + CoW PFNMAP improvements

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Message-ID: <e7a6b0de-3f2a-4584-bc77-078569f69f55@redhat.com>
Date: Tue, 17 Jun 2025 18:18:10 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
 nvdimm@...ts.linux.dev, Andrew Morton <akpm@...ux-foundation.org>,
 Juergen Gross <jgross@...e.com>, Stefano Stabellini
 <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 Dan Williams <dan.j.williams@...el.com>, Alistair Popple
 <apopple@...dia.com>, Matthew Wilcox <willy@...radead.org>,
 Jan Kara <jack@...e.cz>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Zi Yan <ziy@...dia.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.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>, Vlastimil Babka <vbabka@...e.cz>,
 Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>
Subject: Re: [PATCH RFC 00/14] mm: vm_normal_page*() + CoW PFNMAP improvements

On 17.06.25 17:43, David Hildenbrand wrote:
> RFC because it's based on mm-new where some things might still change
> around the devmap removal stuff.
> 
> While removing support for CoW PFNMAPs is a noble goal, I am not even sure
> if we can remove said support for e.g., /dev/mem that easily.
> 
> In the end, Cow PFNMAPs are pretty simple: everything is "special" except
> CoW'ed anon folios, that are "normal".
> 
> The only complication is: how to identify such pages without pte_special().
> Because with pte_special(), it's easy.
> 
> Well, of course, one day all architectures might support pte_special() ...
> either because we added support for pte_special() or removed support for
> ... these architectures from Linux.
> 
> No need to wait for that day. Let's do some cleanups around
> vm_normal_page()/vm_normal_page_pmd() and handling of the huge zero folio,
> and remove the "horrible special case to handle copy-on-write behaviour"
> that does questionable things in remap_pfn_range() with a VMA, simply by
> 
> ... looking for anonymous folios in CoW PFNMAPs to identify anonymous
> folios? I know, sounds crazy ;)

I'll mention one corner case that just occurred to me: assume someone 
maps arbitrary /dev/mem that is actually used by the kernel for user 
space, and then some of that memory gets allocated as anonymous memory, 
it would probably be a problem.

Hmm, I'll have to think about that, and the interaction with 
CONFIG_STRICT_DEVMEM.

-- 
Cheers,

David / dhildenb