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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 X-Rspamd-Queue-Id: EC731C0005 X-Stat-Signature: 14fmn916e1jem4sq1rwz3pmoz8h4651a X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1757323873-799952 X-HE-Meta: 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 ezld4d00 zgN1H X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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