From 5ab246749569cff9f815618f02ba0d7cf20e5edd Mon Sep 17 00:00:00 2001 From: Kevin Brodsky Date: Mon, 15 Dec 2025 15:03:18 +0000 Subject: mm: enable lazy_mmu sections to nest MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 call to leave() disables the batching optimisation before the outer section ends. This patch actually enables lazy_mmu sections to nest by tracking the nesting level in task_struct, in a similar fashion to e.g. pagefault_{enable,disable}(). This is fully handled by the generic lazy_mmu helpers that were recently introduced. lazy_mmu sections were not initially intended to nest, so we need to clarify the semantics w.r.t. the arch_*_lazy_mmu_mode() callbacks. This patch takes the following approach: * The outermost calls to lazy_mmu_mode_{enable,disable}() trigger calls to arch_{enter,leave}_lazy_mmu_mode() - this is unchanged. * Nested calls to lazy_mmu_mode_{enable,disable}() are not forwarded to the arch via arch_{enter,leave} - lazy MMU remains enabled so the assumption is that these callbacks are not relevant. However, existing code may rely on a call to disable() to flush any batched state, regardless of nesting. arch_flush_lazy_mmu_mode() is therefore called in that situation. A separate interface was recently introduced to temporarily pause the lazy MMU mode: lazy_mmu_mode_{pause,resume}(). pause() fully exits the mode *regardless of the nesting level*, and resume() restores the mode at the same nesting level. pause()/resume() are themselves allowed to nest, so we actually store two nesting levels in task_struct: enable_count and pause_count. A new helper is_lazy_mmu_mode_active() is introduced to determine whether we are currently in lazy MMU mode; this will be used in subsequent patches to replace the various ways arch's currently track whether the mode is enabled. In summary (enable/pause represent the values *after* the call): lazy_mmu_mode_enable() -> arch_enter() enable=1 pause=0 lazy_mmu_mode_enable() -> ΓΈ enable=2 pause=0 lazy_mmu_mode_pause() -> arch_leave() enable=2 pause=1 lazy_mmu_mode_resume() -> arch_enter() enable=2 pause=0 lazy_mmu_mode_disable() -> arch_flush() enable=1 pause=0 lazy_mmu_mode_disable() -> arch_leave() enable=0 pause=0 Note: is_lazy_mmu_mode_active() is added to to allow arch headers included by to use it. Link: https://lkml.kernel.org/r/20251215150323.2218608-10-kevin.brodsky@arm.com Signed-off-by: Kevin Brodsky Acked-by: David Hildenbrand (Red Hat) Reviewed-by: Yeoreum Yun Cc: Alexander Gordeev Cc: Andreas Larsson Cc: Anshuman Khandual Cc: Borislav Betkov Cc: Boris Ostrovsky Cc: Catalin Marinas Cc: Christophe Leroy Cc: David Hildenbrand Cc: David S. Miller Cc: David Woodhouse Cc: "H. Peter Anvin" Cc: Ingo Molnar Cc: Jann Horn Cc: Juegren Gross Cc: Liam Howlett Cc: Lorenzo Stoakes Cc: Madhavan Srinivasan Cc: Michael Ellerman Cc: Michal Hocko Cc: Mike Rapoport Cc: Nicholas Piggin Cc: Peter Zijlstra Cc: Ritesh Harjani (IBM) Cc: Ryan Roberts Cc: Suren Baghdasaryan Cc: Thomas Gleinxer Cc: Venkat Rao Bagalkote Cc: Vlastimil Babka Cc: Will Deacon Signed-off-by: Andrew Morton --- include/linux/mm_types_task.h | 5 ++ include/linux/pgtable.h | 114 +++++++++++++++++++++++++++++++++++++++--- include/linux/sched.h | 45 +++++++++++++++++ 3 files changed, 157 insertions(+), 7 deletions(-) (limited to 'include') diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h index a82aa80c0ba4..11bf319d78ec 100644 --- a/include/linux/mm_types_task.h +++ b/include/linux/mm_types_task.h @@ -88,4 +88,9 @@ struct tlbflush_unmap_batch { #endif }; +struct lazy_mmu_state { + u8 enable_count; + u8 pause_count; +}; + #endif /* _LINUX_MM_TYPES_TASK_H */ diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h index dddde6873d1e..2f0dd3a4ace1 100644 --- a/include/linux/pgtable.h +++ b/include/linux/pgtable.h @@ -236,39 +236,139 @@ static inline int pmd_dirty(pmd_t pmd) * The mode is disabled in interrupt context and calls to the lazy_mmu API have * no effect. * - * Nesting is not permitted. + * The lazy MMU mode is enabled for a given block of code using: + * + * lazy_mmu_mode_enable(); + * + * lazy_mmu_mode_disable(); + * + * Nesting is permitted: may itself use an enable()/disable() pair. + * A nested call to enable() has no functional effect; however disable() causes + * any batched architectural state to be flushed regardless of nesting. After a + * call to disable(), the caller can therefore rely on all previous page table + * modifications to have taken effect, but the lazy MMU mode may still be + * enabled. + * + * In certain cases, it may be desirable to temporarily pause the lazy MMU mode. + * This can be done using: + * + * lazy_mmu_mode_pause(); + * + * lazy_mmu_mode_resume(); + * + * pause() ensures that the mode is exited regardless of the nesting level; + * resume() re-enters the mode at the same nesting level. Any call to the + * lazy_mmu_mode_* API between those two calls has no effect. In particular, + * this means that pause()/resume() pairs may nest. + * + * is_lazy_mmu_mode_active() can be used to check whether the lazy MMU mode is + * currently enabled. */ #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE +/** + * lazy_mmu_mode_enable() - Enable the lazy MMU mode. + * + * Enters a new lazy MMU mode section; if the mode was not already enabled, + * enables it and calls arch_enter_lazy_mmu_mode(). + * + * Must be paired with a call to lazy_mmu_mode_disable(). + * + * Has no effect if called: + * - While paused - see lazy_mmu_mode_pause() + * - In interrupt context + */ static inline void lazy_mmu_mode_enable(void) { - if (in_interrupt()) + struct lazy_mmu_state *state = ¤t->lazy_mmu_state; + + if (in_interrupt() || state->pause_count > 0) return; - arch_enter_lazy_mmu_mode(); + VM_WARN_ON_ONCE(state->enable_count == U8_MAX); + + if (state->enable_count++ == 0) + arch_enter_lazy_mmu_mode(); } +/** + * lazy_mmu_mode_disable() - Disable the lazy MMU mode. + * + * Exits the current lazy MMU mode section. If it is the outermost section, + * disables the mode and calls arch_leave_lazy_mmu_mode(). Otherwise (nested + * section), calls arch_flush_lazy_mmu_mode(). + * + * Must match a call to lazy_mmu_mode_enable(). + * + * Has no effect if called: + * - While paused - see lazy_mmu_mode_pause() + * - In interrupt context + */ static inline void lazy_mmu_mode_disable(void) { - if (in_interrupt()) + struct lazy_mmu_state *state = ¤t->lazy_mmu_state; + + if (in_interrupt() || state->pause_count > 0) return; - arch_leave_lazy_mmu_mode(); + VM_WARN_ON_ONCE(state->enable_count == 0); + + if (--state->enable_count == 0) + arch_leave_lazy_mmu_mode(); + else /* Exiting a nested section */ + arch_flush_lazy_mmu_mode(); + } +/** + * lazy_mmu_mode_pause() - Pause the lazy MMU mode. + * + * Pauses the lazy MMU mode; if it is currently active, disables it and calls + * arch_leave_lazy_mmu_mode(). + * + * Must be paired with a call to lazy_mmu_mode_resume(). Calls to the + * lazy_mmu_mode_* API have no effect until the matching resume() call. + * + * Has no effect if called: + * - While paused (inside another pause()/resume() pair) + * - In interrupt context + */ static inline void lazy_mmu_mode_pause(void) { + struct lazy_mmu_state *state = ¤t->lazy_mmu_state; + if (in_interrupt()) return; - arch_leave_lazy_mmu_mode(); + VM_WARN_ON_ONCE(state->pause_count == U8_MAX); + + if (state->pause_count++ == 0 && state->enable_count > 0) + arch_leave_lazy_mmu_mode(); } +/** + * lazy_mmu_mode_resume() - Resume the lazy MMU mode. + * + * Resumes the lazy MMU mode; if it was active at the point where the matching + * call to lazy_mmu_mode_pause() was made, re-enables it and calls + * arch_enter_lazy_mmu_mode(). + * + * Must match a call to lazy_mmu_mode_pause(). + * + * Has no effect if called: + * - While paused (inside another pause()/resume() pair) + * - In interrupt context + */ static inline void lazy_mmu_mode_resume(void) { + struct lazy_mmu_state *state = ¤t->lazy_mmu_state; + if (in_interrupt()) return; - arch_enter_lazy_mmu_mode(); + VM_WARN_ON_ONCE(state->pause_count == 0); + + if (--state->pause_count == 0 && state->enable_count > 0) + arch_enter_lazy_mmu_mode(); } #else static inline void lazy_mmu_mode_enable(void) {} diff --git a/include/linux/sched.h b/include/linux/sched.h index da0133524d08..6b563d4e68f6 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1419,6 +1419,10 @@ struct task_struct { struct page_frag task_frag; +#ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE + struct lazy_mmu_state lazy_mmu_state; +#endif + #ifdef CONFIG_TASK_DELAY_ACCT struct task_delay_info *delays; #endif @@ -1702,6 +1706,47 @@ static inline char task_state_to_char(struct task_struct *tsk) return task_index_to_char(task_state_index(tsk)); } +#ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE +/** + * __task_lazy_mmu_mode_active() - Test the lazy MMU mode state for a task. + * @tsk: The task to check. + * + * Test whether @tsk has its lazy MMU mode state set to active (i.e. enabled + * and not paused). + * + * This function only considers the state saved in task_struct; to test whether + * current actually is in lazy MMU mode, is_lazy_mmu_mode_active() should be + * used instead. + * + * This function is intended for architectures that implement the lazy MMU + * mode; it must not be called from generic code. + */ +static inline bool __task_lazy_mmu_mode_active(struct task_struct *tsk) +{ + struct lazy_mmu_state *state = &tsk->lazy_mmu_state; + + return state->enable_count > 0 && state->pause_count == 0; +} + +/** + * is_lazy_mmu_mode_active() - Test whether we are currently in lazy MMU mode. + * + * Test whether the current context is in lazy MMU mode. This is true if both: + * 1. We are not in interrupt context + * 2. Lazy MMU mode is active for the current task + * + * This function is intended for architectures that implement the lazy MMU + * mode; it must not be called from generic code. + */ +static inline bool is_lazy_mmu_mode_active(void) +{ + if (in_interrupt()) + return false; + + return __task_lazy_mmu_mode_active(current); +} +#endif + extern struct pid *cad_pid; /* -- cgit v1.2.3