From: "David Hildenbrand (Arm)" <david@kernel.org>
To: Lorenzo Stoakes <ljs@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
"Liam R. Howlett" <liam@infradead.org>,
Vlastimil Babka <vbabka@kernel.org>,
Mike Rapoport <rppt@kernel.org>,
Suren Baghdasaryan <surenb@google.com>,
Michal Hocko <mhocko@suse.com>,
Oscar Salvador <osalvador@suse.de>,
Hugh Dickins <hughd@google.com>,
Lance Yang <lance.yang@linux.dev>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Bibo Mao <maobibo@loongson.cn>,
stable@vger.kernel.org
Subject: Re: [PATCH] mm: fix __vm_normal_page() to handle missing support for pmd_special()/pud_special()
Date: Tue, 5 May 2026 16:36:16 +0200 [thread overview]
Message-ID: <31e04da5-89ff-4efe-9bbc-6845e4ffc45e@kernel.org> (raw)
In-Reply-To: <afnffbs-5cy7KGua@lucifer>
On 5/5/26 14:20, Lorenzo Stoakes wrote:
> On Thu, Apr 30, 2026 at 01:31:22PM +0200, David Hildenbrand (Arm) wrote:
>> On x86 32-bit with THP enabled, zap_huge_pmd() is seen to generate a
>> "WARNING: mm/memory.c:735 at __vm_normal_page+0x6a/0x7d", from the
>> VM_WARN_ON_ONCE(is_zero_pfn(pfn) || is_huge_zero_pfn(pfn)); followed
>> by "BUG: Bad rss-counter state"s, then later "BUG: Bad page state"s
>> when reclaim gets to call shrink_huge_zero_folio_scan().
>>
>> It's as if the _PAGE_SPECIAL bit never got set in the huge_zero pmd:
>> and indeed, whereas pte_special() and pte_mkspecial() are subject to a
>> dedicated CONFIG_ARCH_HAS_PTE_SPECIAL, pmd_special() and pmd_mkspecial()
>> are subject to CONFIG_ARCH_SUPPORTS_PMD_PFNMAP, which is never enabled
>> on any 32-bit architecture.
>
> Ah damn. I wonder if it's really a combination of 'supports THP' and 'has a
> spare software defined bit free in PTE'?
>
I think it's mostly "we have a spare bit (pte_special() implemented), but we
didn't wire up pmd_special()"
Thanks!
--
Cheers,
David
next prev parent reply other threads:[~2026-05-05 14:36 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-04-30 11:31 [PATCH] mm: fix __vm_normal_page() to handle missing support for pmd_special()/pud_special() David Hildenbrand (Arm)
2026-05-05 12:20 ` Lorenzo Stoakes
2026-05-05 14:36 ` David Hildenbrand (Arm) [this message]
2026-05-06 4:17 ` Oscar Salvador
2026-05-06 5:11 ` Baolin Wang
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