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

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Message-ID: <aad59217-0763-473e-bdc8-d0d47bec35bf@redhat.com>
Date: Fri, 11 Jul 2025 20:57:33 +0200
From: David Hildenbrand <david@...hat.com>
To: Hugh Dickins <hughd@...gle.com>
Cc: Lance Yang <lance.yang@...ux.dev>, 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 11.07.25 20:49, Hugh Dickins wrote:
> On Fri, 11 Jul 2025, David Hildenbrand wrote:
>> On 08.07.25 04:52, Hugh Dickins wrote:
>>>
>>> Of course it's limited in what it can catch (and won't even get called
>>> if the present bit was not set - a more complete patch might unify with
>>> those various "Bad swap" messages). Of course. But it's still useful for
>>> stopping pfn_to_page() veering off the end of the memmap[] (in some
>>> configs).
>>
>> Right, probably in the configs we both don't care that much about nowadays :)
> 
> I thought it was the other way round: it's useful for stopping
> pfn_to_page() veering off the end of the memmap[] if it's a memory model
> where pfn_to_page() is a simple linear conversion.
> 
> As with SPARSEMEM_VMEMMAP, which I thought was the favourite nowadays.

Yes, you're right, I had the !SPARSEMEM model in mind, but obviously, 
the same applies for the SPARSEMEM_VMEMMAP case as well.

Only the SPARSEMEM && !SPARSEMEM_VMEMMAP model is weird. IIRC, it will 
dereference NULL when given a non-existant PFN. (__nr_to_section 
returning NULL and pfn_to_section_nr() not beeing happy about that)

-- 
Cheers,

David / dhildenb