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Miller" , "H. Peter Anvin" , Ingo Molnar , Jann Horn , Juergen Gross , "Liam R. Howlett" , Lorenzo Stoakes , Madhavan Srinivasan , Michael Ellerman , Michal Hocko , Mike Rapoport , Nicholas Piggin , Peter Zijlstra , Ryan Roberts , Suren Baghdasaryan , Thomas Gleixner , Vlastimil Babka , Will Deacon , linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, sparclinux@vger.kernel.org, xen-devel@lists.xenproject.org Subject: Re: [PATCH v2 3/7] arm64: mm: fully support nested lazy_mmu sections Message-ID: References: <20250908073931.4159362-1-kevin.brodsky@arm.com> <20250908073931.4159362-4-kevin.brodsky@arm.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250908073931.4159362-4-kevin.brodsky@arm.com> X-ClientProxiedBy: LO2P123CA0080.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:138::13) To GV1PR08MB10521.eurprd08.prod.outlook.com (2603:10a6:150:163::20) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-TrafficTypeDiagnostic: GV1PR08MB10521:EE_|DB5PR08MB10163:EE_|AM4PEPF00027A5F:EE_|DB9PR08MB7771:EE_ X-MS-Office365-Filtering-Correlation-Id: 06fcb68d-f2eb-4a24-3b41-08ddeeba6fe9 x-checkrecipientrouted: true NoDisclaimer: true X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam-Untrusted: BCL:0;ARA:13230040|1800799024|376014|7416014|366016; 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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 08 Sep 2025 09:31:06.4414 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 06fcb68d-f2eb-4a24-3b41-08ddeeba6fe9 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AM4PEPF00027A5F.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB9PR08MB7771 Reviewed-by: Yeoreum Yun On Mon, Sep 08, 2025 at 08:39:27AM +0100, Kevin Brodsky wrote: > Despite recent efforts to prevent lazy_mmu sections from nesting, it > remains difficult to ensure that it never occurs - and in fact it > does occur on arm64 in certain situations (CONFIG_DEBUG_PAGEALLOC). > Commit 1ef3095b1405 ("arm64/mm: Permit lazy_mmu_mode to be nested") > made nesting tolerable on arm64, but without truly supporting it: > the inner leave() call clears TIF_LAZY_MMU, disabling the batching > optimisation before the outer section ends. > > Now that the lazy_mmu API allows enter() to pass through a state to > the matching leave() call, we can actually support nesting. If > enter() is called inside an active lazy_mmu section, TIF_LAZY_MMU > will already be set, and we can then return LAZY_MMU_NESTED to > instruct the matching leave() call not to clear TIF_LAZY_MMU. > > The only effect of this patch is to ensure that TIF_LAZY_MMU (and > therefore the batching optimisation) remains set until the outermost > lazy_mmu section ends. leave() still emits barriers if needed, > regardless of the nesting level, as the caller may expect any > page table changes to become visible when leave() returns. > > Signed-off-by: Kevin Brodsky > --- > arch/arm64/include/asm/pgtable.h | 19 +++++-------------- > 1 file changed, 5 insertions(+), 14 deletions(-) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index 816197d08165..602feda97dc4 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -85,24 +85,14 @@ typedef int lazy_mmu_state_t; > > static inline lazy_mmu_state_t arch_enter_lazy_mmu_mode(void) > { > - /* > - * lazy_mmu_mode is not supposed to permit nesting. But in practice this > - * does happen with CONFIG_DEBUG_PAGEALLOC, where a page allocation > - * inside a lazy_mmu_mode section (such as zap_pte_range()) will change > - * permissions on the linear map with apply_to_page_range(), which > - * re-enters lazy_mmu_mode. So we tolerate nesting in our > - * implementation. The first call to arch_leave_lazy_mmu_mode() will > - * flush and clear the flag such that the remainder of the work in the > - * outer nest behaves as if outside of lazy mmu mode. This is safe and > - * keeps tracking simple. > - */ > + int lazy_mmu_nested; > > if (in_interrupt()) > return LAZY_MMU_DEFAULT; > > - set_thread_flag(TIF_LAZY_MMU); > + lazy_mmu_nested = test_and_set_thread_flag(TIF_LAZY_MMU); > > - return LAZY_MMU_DEFAULT; > + return lazy_mmu_nested ? LAZY_MMU_NESTED : LAZY_MMU_DEFAULT; > } > > static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state) > @@ -113,7 +103,8 @@ static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state) > if (test_and_clear_thread_flag(TIF_LAZY_MMU_PENDING)) > emit_pte_barriers(); > > - clear_thread_flag(TIF_LAZY_MMU); > + if (state != LAZY_MMU_NESTED) > + clear_thread_flag(TIF_LAZY_MMU); > } > > #ifdef CONFIG_TRANSPARENT_HUGEPAGE > -- > 2.47.0 > -- Sincerely, Yeoreum Yun