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

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Message-ID: <aFVZCvOpIpBGAf9w@localhost.localdomain>
Date: Fri, 20 Jun 2025 14:50:18 +0200
From: Oscar Salvador <osalvador@...e.de>
To: David Hildenbrand <david@...hat.com>
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 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>

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?
 

-- 
Oscar Salvador
SUSE Labs