[RFC PATCH 06/13] mm/kwatch: add lockless per-task context pool
From: Jinchao Wang <hidden>
Date: 2026-07-14 18:31:38
Also in:
linux-doc, linux-mm, linux-perf-users, lkml
Subsystem:
memory management, the rest · Maintainers:
Andrew Morton, Linus Torvalds
A task that enters the watched function needs somewhere to keep its window state (nesting depth, owned watchpoint, config epoch). The lookup runs in kprobe and NMI-like contexts, so it must not allocate or take locks. Use a preallocated open-addressing array hashed by task_struct pointer. Slots are claimed with cmpxchg() and released with smp_store_release(); lookup is a read-only probe sequence. The pool size (max_concurrency) bounds how many tasks can be inside watch windows concurrently; excess tasks are simply not tracked. Signed-off-by: Jinchao Wang <redacted> --- mm/kwatch/Makefile | 2 +- mm/kwatch/task_ctx.c | 105 +++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 106 insertions(+), 1 deletion(-) create mode 100644 mm/kwatch/task_ctx.c
diff --git a/mm/kwatch/Makefile b/mm/kwatch/Makefile
index 69c21ae62123..cc6574df0d68 100644
--- a/mm/kwatch/Makefile
+++ b/mm/kwatch/Makefile@@ -1,3 +1,3 @@ obj-$(CONFIG_KWATCH) += kwatch.o -kwatch-y := deref.o +kwatch-y := deref.o task_ctx.o
diff --git a/mm/kwatch/task_ctx.c b/mm/kwatch/task_ctx.c
new file mode 100644
index 000000000000..f8e582f0dcfe
--- /dev/null
+++ b/mm/kwatch/task_ctx.c@@ -0,0 +1,105 @@ +// SPDX-License-Identifier: GPL-2.0 +#include <linux/slab.h> +#include <linux/hash.h> +#include <linux/sched.h> +#include <linux/log2.h> +#include "kwatch.h" + +static u16 kwatch_ctx_pool_size; +static u16 kwatch_ctx_pool_mask; + +static struct kwatch_tsk_ctx *kwatch_ctx_pool; + +int kwatch_tsk_ctx_prealloc(u16 max_concurrency) +{ + if (!max_concurrency) + max_concurrency = 256; + + kwatch_ctx_pool_size = roundup_pow_of_two(max_concurrency); + kwatch_ctx_pool_mask = kwatch_ctx_pool_size - 1; + + if (unlikely(!kwatch_ctx_pool)) { + kwatch_ctx_pool = kcalloc(kwatch_ctx_pool_size, + sizeof(struct kwatch_tsk_ctx), + GFP_KERNEL); + if (!kwatch_ctx_pool) + return -ENOMEM; + } + return 0; +} + +struct kwatch_tsk_ctx *kwatch_tsk_ctx_get(bool can_alloc) +{ + int start_idx, i, idx; + struct task_struct *t; + + if (unlikely(!kwatch_ctx_pool)) + return NULL; + + start_idx = hash_ptr(current, ilog2(kwatch_ctx_pool_size)); + + for (i = 0; i < kwatch_ctx_pool_size; i++) { + idx = (start_idx + i) & kwatch_ctx_pool_mask; + t = READ_ONCE(kwatch_ctx_pool[idx].task); + if (t == current) + return &kwatch_ctx_pool[idx]; + } + + if (!can_alloc) + return NULL; + + for (i = 0; i < kwatch_ctx_pool_size; i++) { + idx = (start_idx + i) & kwatch_ctx_pool_mask; + t = READ_ONCE(kwatch_ctx_pool[idx].task); + if (!t) { + if (!cmpxchg(&kwatch_ctx_pool[idx].task, NULL, current)) + return &kwatch_ctx_pool[idx]; + } + } + + return NULL; +} + +void kwatch_tsk_ctx_reset(struct kwatch_tsk_ctx *ctx, u32 new_epoch) +{ + struct kwatch_watchpoint *wp = xchg(&ctx->wp, NULL); + + if (wp) + kwatch_hwbp_put(wp); + ctx->depth = 0; + ctx->epoch = new_epoch; +} + +void kwatch_tsk_ctx_put(void) +{ + struct kwatch_tsk_ctx *ctx = kwatch_tsk_ctx_get(false); + + if (unlikely(!ctx)) + return; + + kwatch_tsk_ctx_reset(ctx, 0); + + /* Pairs with READ_ONCE() in kwatch_tsk_ctx_get() */ + smp_store_release(&ctx->task, NULL); +} + +void kwatch_tsk_ctx_release_wps(void) +{ + int i; + + if (!kwatch_ctx_pool) + return; + + for (i = 0; i < kwatch_ctx_pool_size; i++) { + struct kwatch_watchpoint *wp = xchg(&kwatch_ctx_pool[i].wp, + NULL); + if (wp) + kwatch_hwbp_put(wp); + } +} + +void kwatch_tsk_ctx_free(void) +{ + kfree(kwatch_ctx_pool); + kwatch_ctx_pool = NULL; +}
--
2.53.0