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: <155d1f58-6568-4efa-968e-af3873707ad0@redhat.com>
Date: Mon, 23 Jun 2025 21:19:53 +0200
From: David Hildenbrand <david@...hat.com>
To: Pedro Falcato <pfalcato@...e.de>
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 20.06.25 20:24, Pedro Falcato wrote:
> 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?

In theory yes, in practice I think this will not happen.

... especially because the WARN_ON_ONCE() would already trigger in many 
other cases before we would find one situation where it doesn't.

That's why I decided against Fixes: and declaring this more a cleanup, 
because the starts really would have to align ... :)

Thanks!

-- 
Cheers,

David / dhildenb