linux-kernel - Re: [PATCH RFC 01/14] mm/memory: drop highest_memmap_pfn sanity check in vm_normal

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <b6d79033-b887-4ce7-b8f2-564cad7ec535@redhat.com>
Date: Fri, 4 Jul 2025 14:40:25 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: Oscar Salvador <osalvador@...e.de>, 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>,
 Pedro Falcato <pfalcato@...e.de>, Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH RFC 01/14] mm/memory: drop highest_memmap_pfn sanity check
 in vm_normal_page()

On 03.07.25 16:44, Lance Yang wrote:
> 
> 
> On 2025/7/3 20:39, David Hildenbrand wrote:
>> On 03.07.25 14:34, Lance Yang wrote:
>>> On Mon, Jun 23, 2025 at 10:04 PM David Hildenbrand <david@...hat.com>
>>> wrote:
>>>>
>>>> On 20.06.25 14:50, Oscar Salvador wrote:
>>>>> On Tue, Jun 17, 2025 at 05:43:32PM +0200, David Hildenbrand wrote:
>>>>>> In 2009, we converted a VM_BUG_ON(!pfn_valid(pfn)) to the current
>>>>>> highest_memmap_pfn sanity check in commit 22b31eec63e5 ("badpage:
>>>>>> vm_normal_page use print_bad_pte"), because highest_memmap_pfn was
>>>>>> readily available.
>>>>>>
>>>>>> Nowadays, this is the last remaining highest_memmap_pfn user, and this
>>>>>> sanity check is not really triggering ... frequently.
>>>>>>
>>>>>> Let's convert it to VM_WARN_ON_ONCE(!pfn_valid(pfn)), so we can
>>>>>> simplify and get rid of highest_memmap_pfn. Checking for
>>>>>> pfn_to_online_page() might be even better, but it would not handle
>>>>>> ZONE_DEVICE properly.
>>>>>>
>>>>>> Do the same in vm_normal_page_pmd(), where we don't even report a
>>>>>> problem at all ...
>>>>>>
>>>>>> What might be better in the future is having a runtime option like
>>>>>> page-table-check to enable such checks dynamically on-demand.
>>>>>> Something
>>>>>> for the future.
>>>>>>
>>>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>>>>
>>>>
>>>> Hi Oscar,
>>>>
>>>>> I'm confused, I'm missing something here.
>>>>> Before this change we would return NULL if e.g: pfn >
>>>>> highest_memmap_pfn, but
>>>>> now we just print the warning and call pfn_to_page() anyway.
>>>>> AFAIK, pfn_to_page() doesn't return NULL?
>>>>
>>>> You're missing that vm_normal_page_pmd() was created as a copy from
>>>> vm_normal_page() [history of the sanity check above], but as we don't
>>>> have (and shouldn't have ...) print_bad_pmd(), we made the code look
>>>> like this would be something that can just happen.
>>>>
>>>> "
>>>> Do the same in vm_normal_page_pmd(), where we don't even report a
>>>> problem at all ...
>>>> "
>>>>
>>>> So we made something that should never happen a runtime sanity check
>>>> without ever reporting a problem ...
>>>
>>> IIUC, the reasoning is that because this case should never happen, we can
>>> change the behavior from returning NULL to a "warn and continue" model?
>>
>> Well, yes. Point is, that check should have never been copy-pasted that
>> way, while dropping the actual warning :)
> 
> Ah, I see your point now. Thanks for clarifying!

I'll add some of that to the patch description. Thanks!

-- 
Cheers,

David / dhildenb