From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
To: "David Hildenbrand (Arm)" <david@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Zi Yan <ziy@nvidia.com>,
Baolin Wang <baolin.wang@linux.alibaba.com>,
"Liam R. Howlett" <liam@infradead.org>,
Nico Pache <npache@redhat.com>,
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>,
Pankaj Raghav <p.raghav@samsung.com>,
Hannes Reinecke <hare@suse.de>, Hugh Dickins <hughd@google.com>,
Yang Shi <shy828301@gmail.com>, Kiryl Shutsemau <kas@kernel.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Hengbin Zhang <uqbarz@gmail.com>,
stable@vger.kernel.org
Subject: Re: [PATCH mm-hotfixes 2/2] mm/huge_memory: fix huge_zero_pfn race
Date: Thu, 30 Jul 2026 14:09:17 +0100 [thread overview]
Message-ID: <amtHMG4owrzXeLx0@lucifer> (raw)
In-Reply-To: <39b527d7-9c6e-4798-be9d-b834c8a3d2a0@kernel.org>
On Thu, Jul 30, 2026 at 02:10:23PM +0200, David Hildenbrand (Arm) wrote:
> On 7/30/26 11:55, Lorenzo Stoakes (ARM) wrote:
> > On Thu, Jul 30, 2026 at 11:22:09AM +0200, David Hildenbrand (Arm) wrote:
> >> [...]
> >>
> >>> So invariants are required - huge_zero_refcount MUST:
> >>>
> >>> * Only be set in the huge_zero_lock critical section to ensure
> >>> serialisation of huge_zero_pfn, huge_zero_folio and huge_zero_refcount
> >>> writes.
> >>>
> >>> * Be set non-zero only AFTER huge_zero_[pfn, folio] are set to valid values
> >>> so installation of the huge zero folio on read page fault ensures
> >>> concurrent is_huge_zero_*() calls correctly identify the huge zero folio.
> >>>
> >>> * Be set zero only BEFORE huge_zero_[pfn, folio] are set to NULL and ~0UL
> >>> respectively, and atomically.
> >>>
> >>> Establish these by:
> >>>
> >>> * Only updating huge_zero_refcount in the huge_zero_lock critical section
> >>> in get_huge_zero_folio() and shrink_huge_zero_folio_scan().
> >>
> >> That is imprecise. huge_zero_refcount is updated (incremented) outside of
> >> huge_zero_lock in get_huge_zero_folio().
> >
> > You're right.
> >
> > ->
> >
> > * Only setting huge_zero_refcount to zero or an absolute value in the
> > huge_zero_lock critical section in get_huge_zero_folio() and
> > shrink_huge_zero_folio_scan() and updating atomically elsewhere.
> >
> > Seems better?
>
> Yes, I guess the transition from/to 0 are the problematic bits. Everything else
> can just be lockless.
Yeah exactly. Also setting an absolute value in general is potentially
problematic.
>
> --
> Cheers,
>
> David
Cheers, Lorenzo
next prev parent reply other threads:[~2026-07-30 13:09 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-07-28 12:05 [PATCH mm-hotfixes 0/2] mm/huge_memory: fix huge_zero_pfn race Lorenzo Stoakes (ARM)
2026-07-28 12:05 ` [PATCH mm-hotfixes 1/2] mm/huge_memory: separate out CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic Lorenzo Stoakes (ARM)
2026-07-28 15:08 ` Kiryl Shutsemau
2026-07-28 15:41 ` Lorenzo Stoakes (ARM)
2026-07-28 12:05 ` [PATCH mm-hotfixes 2/2] mm/huge_memory: fix huge_zero_pfn race Lorenzo Stoakes (ARM)
2026-07-30 9:22 ` David Hildenbrand (Arm)
2026-07-30 9:55 ` Lorenzo Stoakes (ARM)
2026-07-30 12:10 ` David Hildenbrand (Arm)
2026-07-30 13:09 ` Lorenzo Stoakes (ARM) [this message]
2026-07-28 19:02 ` [PATCH mm-hotfixes 0/2] " David Hildenbrand (Arm)
2026-07-29 8:15 ` Lorenzo Stoakes (ARM)
2026-07-30 0:55 ` Andrew Morton
2026-07-30 9:05 ` David Hildenbrand (Arm)
2026-07-30 9:15 ` Lorenzo Stoakes (ARM)
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