From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
To: "David Hildenbrand (Arm)" <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>,
Shuah Khan <skhan@linuxfoundation.org>,
Randy Dunlap <rdunlap@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
"Liam R. Howlett" <liam@infradead.org>,
Vlastimil Babka <vbabka@kernel.org>,
Jann Horn <jannh@google.com>, Pedro Falcato <pfalcato@suse.de>,
Mike Rapoport <rppt@kernel.org>,
Suren Baghdasaryan <surenb@google.com>,
Michal Hocko <mhocko@suse.com>, Peter Xu <peterx@redhat.com>,
Steven Rostedt <rostedt@goodmis.org>,
Masami Hiramatsu <mhiramat@kernel.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Zi Yan <ziy@nvidia.com>,
Baolin Wang <baolin.wang@linux.alibaba.com>,
Nico Pache <nico.pache@linux.dev>,
Ryan Roberts <ryan.roberts@arm.com>, Dev Jain <dev.jain@arm.com>,
Barry Song <baohua@kernel.org>,
Lance Yang <lance.yang@linux.dev>,
Usama Arif <usama.arif@linux.dev>,
Kiryl Shutsemau <kas@kernel.org>,
Axel Rasmussen <axelrasmussen@google.com>,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-doc@vger.kernel.org, linux-mm@kvack.org,
linux-trace-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] mm/userfaultfd: prevent poison marker loss
Date: Sat, 14 Nov 2026 17:58:40 +0000 [thread overview]
Message-ID: <avihSinN3BWkw2sS@gremlin> (raw)
In-Reply-To: <822c1c7e-bfdf-4570-bf30-08f187e7253d@kernel.org>
On Sat, Nov 14, 2026 at 04:39:44PM +0100, David Hildenbrand (Arm) wrote:
> On 11/7/26 00:51, David Hildenbrand (Arm) wrote:
> > Unregistering userfaultfd leaves poison markers in the page tables.
> > However, once unregistered, hugetlb PMD tables can be shared again.
> >
> > Further, shmem page-table retraction can discard poison markers even
> > while registered in missing mode, and fork() can skip copying page
> > tables containing poison markers.
> >
> > To fix it, add VM_MAYBE_UFFD_POISON and set it before installing poison
> > markers. Preserve it across unregistration, splits, merges and fork(),
> > just like we do with guard markers.
> >
> > Use the new atomic VMA flag to disable PMD sharing and page-table
> > retraction, and to force page-table copying during fork().
> >
> > Move VM_UFFD_WP to bit 44 and make HAVE_ARCH_USERFAULTFD_WP depend on
> > 64BIT, freeing bit 12 for the new flag on both 32-bit and 64-bit systems.
> > Expose the flag as "up" in smaps.
> >
> > Losing poison markers, while unpleasant, is not really fatal, and the
> > use case is rather a niche thing. In addition, we rely on the atomic
> > VMA flags infrastructure ... so let's not CC stable.
> >
> > Fixes: fc71884a5f59 ("mm: userfaultfd: add new UFFDIO_POISON ioctl")
> > Fixes: 8a13897fb0da ("mm: userfaultfd: support UFFDIO_POISON for hugetlbfs")
> > Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> > ---
>
> Ping on this one.
Ack, will have a look.
>
> --
> Cheers,
>
> David
--
Cheers, Lorenzo
next prev parent reply other threads:[~2026-11-14 17:58 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-06 23:51 [PATCH 0/2] mm/userfaultfd: prevent poison marker loss David Hildenbrand (Arm)
2026-11-06 23:51 ` [PATCH 1/2] " David Hildenbrand (Arm)
2026-11-14 15:39 ` David Hildenbrand (Arm)
2026-11-14 17:58 ` Lorenzo Stoakes (ARM) [this message]
2026-11-14 18:40 ` Lorenzo Stoakes (ARM)
2026-11-06 23:51 ` [PATCH 2/2] mm/khugepaged: check for empty PTE tables before retracting David Hildenbrand (Arm)
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