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

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Message-ID: <c88c29d2-d887-4c5a-8b4e-0cf30e71d596@redhat.com>
Date: Mon, 23 Jun 2025 16:04:01 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...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>,
 Pedro Falcato <pfalcato@...e.de>
Subject: Re: [PATCH RFC 01/14] mm/memory: drop highest_memmap_pfn sanity check
 in vm_normal_page()

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 ...

-- 
Cheers,

David / dhildenb