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

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Message-ID: <aa977869-f93f-4c2b-a189-f90e2d3bc7da@linux.dev>
Date: Thu, 3 Jul 2025 22:44:44 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>
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 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!

> 
> It's a sanity check for something that should never happen, turned into 
> something that looks like it must be handled and would be valid to 
> encounter.

Yeah. Makes sense to me ;)