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

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Message-ID: <5e439af4-6281-43b2-cbd2-616f5d115fdf@google.com>
Date: Fri, 11 Jul 2025 11:49:37 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: David Hildenbrand <david@...hat.com>
cc: Hugh Dickins <hughd@...gle.com>, 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 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.

If you don't care about that one much (does hotplug prevent it?), then
you do care about the complex pfn_to_page()s, and we should have worried
more when "page++"s got unnecessarily converted to folio_page(folio, i)
a year or two back (I'm thinking of in mm/rmap.c, maybe elsewhere).

Hugh