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Date: Wed, 24 Jun 2026 21:42:30 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Dev Jain <dev.jain@arm.com>
Cc: david@kernel.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org,
 vbabka@kernel.org, harry@kernel.org, jannh@google.com, kas@kernel.org,
 linux-mm@kvack.org, linux-kernel@vger.kernel.org, ryan.roberts@arm.com,
 anshuman.khandual@arm.com, stable@vger.kernel.org
Subject: Re: [PATCH] mm/rmap: use huge_ptep_get() in try_to_unmap_one()
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On Thu, 25 Jun 2026 04:28:51 +0000 Dev Jain <dev.jain@arm.com> wrote:

> try_to_unmap_one() handles hugetlb folios when memory failure needs
> to replace a poisoned hugetlb mapping with a hwpoison entry. In that
> case page_vma_mapped_walk() returns the hugetlb entry in pvmw.pte, but
> the code reads it with ptep_get() before decoding the PFN.
> 
> That is wrong on architectures where hugetlb entries are not encoded as
> regular PTEs. On s390, for example, a raw huge RSTE must be converted
> by huge_ptep_get() before helpers such as pte_pfn() can inspect it. A
> raw decode can select the wrong subpage, so try_to_unmap_one() can
> install a hwpoison entry for the wrong PFN.
> 
> The userspace-visible result is that a later access to the poisoned
> hugetlb subpage can miss the expected SIGBUS. With DEBUG_VM, the wrong
> subpage can also trip the PageHWPoison check.
> 
> Use huge_ptep_get() for hugetlb mappings before decoding the PFN.
> 
> Before c7ab0d2fdc84, the bug existed in the form of a plain dereference:
> we would check the head page pfn of the hugetlb with pte_pfn(*pte), and
> bail out on mismatch. This would mean that the hwpoisoned entry will not
> get installed.
> 
> I am not sure what is the procedure on such kinds of very old bugs - how
> back should I really go?

I think 9 years is enough ;)

> There are similar old bugs present, in try_to_migrate_one(), check_pte(),
> remove_migration_pte(), prot_none_hugetlb_entry().

Why now?  Was there some more recent (s390?) change which exposed this?