linux-kernel - Re: [PATCH RFC 03/14] mm: compare pfns only if the entry is present when inserting pfns/pages

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Message-ID: <dq5r2xmw3ypfk2luffas45up525aig4nu7qogojajspukak74o@gtg4kwwvjb5c>
Date: Fri, 20 Jun 2025 19:24:23 +0100
From: Pedro Falcato <pfalcato@...e.de>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, 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>
Subject: Re: [PATCH RFC 03/14] mm: compare pfns only if the entry is present
 when inserting pfns/pages

On Tue, Jun 17, 2025 at 05:43:34PM +0200, David Hildenbrand wrote:
> Doing a pte_pfn() etc. of something that is not a present page table
> entry is wrong. Let's check in all relevant cases where we want to
> upgrade write permissions when inserting pfns/pages whether the entry
> is actually present.
> 
> It's not expected to have caused real harm in practice, so this is more a
> cleanup than a fix for something that would likely trigger in some
> weird circumstances.

Couldn't we e.g have a swap entry's "pfn" accidentally match the one we're
inserting? Isn't that a correctness problem?

> 
> At some point, we should likely unify the two pte handling paths,
> similar to how we did it for pmds/puds.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>

Nice little cleanup, thanks.

Reviewed-by: Pedro Falcato <pfalcato@...e.de>

-- 
Pedro