Hi Abhishek
On 2026/10/8 06:39, Abhishek Bapat wrote: > The memory allocation profiling framework intercepts allocations across > the core subsystems, but currently lacks runtime tracing hooks for > standard observability tools to dynamically track the context (stack > traces and lifecycles of the individual memory chunks) of the > allocations made. > > Introduce three standard trace events to allow this tracking: > > 1. `alloc_tag_hit`: Fired at the exact call site. This allows userspace > tools to trigger and capture a call stack. > 2. `alloc_tag_mem_alloced`: Fired in alloc_tag_add upon successful > allocation. It records the allocated size, the tag, and the uniquely > generated `ptr` value which is `codetag_ref` for slab and percpu > allocators and the pointer to the head `struct page` for the page > allocator. > 3. `alloc_tag_mem_freed`: Fired in alloc_tag_sub right before memory is > freed, yielding the same `ptr` value to allow tracing tools to find > the corresponding allocation. > > Because the introduced trace events occur at different stages in the > call stack, userspace tracing tools must stitch them together to form a > complete picture of a buffer's lifetime. Here's an example of how > userspace correlates these three events: > > 1. On `alloc_tag_hit`: The tool captures the stack trace and caches it, > keyed by the combination of the current thread's PID and the `tag`. > 2. On `alloc_tag_mem_alloced`: The tool extracts the PID and `tag` from > the event and looks up the stack trace cached in step 1. It creates a > new active allocation record, mapping the new provided `ptr` value > to this cached stack trace and the newly returned allocation size. > 3. On `alloc_tag_mem_freed`: When the memory is freed, the event yields > the same `ptr` value. The tool uses this reference to look up the > original allocation record, correlates the free, and safely retires > the tracking entry. > > Modify `alloc_tag_add()` and `alloc_tag_sub()` with a `ptr` argument to > map frees to allocations in the trace events. Slab and percpu pass the > address of the object's `codetag_ref`. The page allocator passes the > head `struct page` because its `codetag_ref` is a temporary copy on the > stack. > > Also, introduce `alloc_tag_trace_key` static key to minimize the > overhead when no tags are being traced (the usual case). Once tracing > for any tag is requested, the key is set, opening the path to check > whether tracing is enabled for the current tag. > Note that the mechanism to enable tag tracing is implemented in the next > patch, therefore for now, `alloc_tag_trace_key` stays always unset. > > Signed-off-by: Abhishek Bapat <[email protected]> Acked-by: Hao Ge <[email protected]> > --- > Documentation/mm/allocation-profiling.rst | 53 ++++++++++ > MAINTAINERS | 1 + > include/linux/alloc_tag.h | 67 +++++++++--- > include/trace/events/alloc_tag.h | 122 ++++++++++++++++++++++ > mm/alloc_tag.c | 28 +++++ > mm/page_alloc.c | 4 +- > mm/percpu.c | 12 ++- > mm/slub.c | 6 +- > 8 files changed, 269 insertions(+), 24 deletions(-) > create mode 100644 include/trace/events/alloc_tag.h > > diff --git a/Documentation/mm/allocation-profiling.rst > b/Documentation/mm/allocation-profiling.rst > index b11ea77c0673..b241450394df 100644 > --- a/Documentation/mm/allocation-profiling.rst > +++ b/Documentation/mm/allocation-profiling.rst > @@ -129,6 +129,59 @@ To do so: > - Then, use the following form for your allocations: > alloc_hooks_tag(ht->your_saved_tag, kmalloc_noprof(...)) > > +Tracing > +======= > + > +Three trace events are available under > `/sys/kernel/tracing/events/alloc_tag` to > +expose the full call stack and the lifetime of individual allocations: > + > +- `alloc_tag_hit`: Fired at the exact call site, before the allocation > happens. > + Can be used to capture the call stack of the caller. > + > +- `alloc_tag_mem_alloced`: Fired once the allocation succeeds. Carries the > `ptr`, > + `tag` and `bytes` for each allocation. > + > +- `alloc_tag_mem_freed`: Fired before memory is freed. Carries the same > + `ptr`, `tag` and `bytes` as the matching alloc event. > + > +`ptr` identifies the allocation, and its meaning depends on the allocator: > + > +- slab and percpu: the address of the object's `codetag_ref` > +- page allocator: the `struct page` pointer of the head page > + > +Correlating the events > +---------------------- > + > +`alloc_tag_hit` and `alloc_tag_mem_alloced` come from different points in the > +call stack, so they have to be stitched together by the user consuming the > +events. Here's a typical flow: > + > +1. On `alloc_tag_hit`: Capture the stack trace and cache it keyed by `(pid, > tag)`. > + > +2. On `alloc_tag_mem_alloced`: Look up the cached stack trace by `(pid, > tag)`, > + then create an active allocation record keyed by `ptr` that holds the > stack > + trace and `bytes`. > + > +3. On `alloc_tag_mem_freed`: Look up the record by `ptr` and retire it. > + > +Limitations > +----------- > + > +- For the page allocator, events are generated for the original allocation > and > + the free only. If the tag reference is split or moved to another folio in > + between, no event is generated for that. As a result: > + > + - a free event may carry a `ptr` that never appeared in an alloc event; > + - `bytes` in a free event may be smaller than in the matching alloc event. > + > + Tools should correlate on `ptr` but must not assume that freed bytes equal > + allocated bytes, or that every free has a matching alloc. > + > +- Freeing a non-compound high-order page with `__free_pages()` while another > CPU > + holds a reference frees the tail pages immediately and the head page later > when > + the reference is dropped. The free event is emitted at the second point and > + reports `PAGE_SIZE` rather than the full size. > + > Notes > ===== > > diff --git a/MAINTAINERS b/MAINTAINERS > index 5c38da7090db..751ce786a378 100644 > --- a/MAINTAINERS > +++ b/MAINTAINERS > @@ -17100,6 +17100,7 @@ S: Maintained > F: Documentation/mm/allocation-profiling.rst > F: include/linux/alloc_tag.h > F: include/linux/pgalloc_tag.h > +F: include/trace/events/alloc_tag.h > F: include/uapi/linux/alloc_tag.h > F: mm/alloc_tag.c > F: tools/testing/selftests/alloc_tag/ > diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h > index 7f2d80a59792..49825177d8c8 100644 > --- a/include/linux/alloc_tag.h > +++ b/include/linux/alloc_tag.h > @@ -128,12 +128,33 @@ DECLARE_PER_CPU(struct alloc_tag_counters, > _shared_alloc_tag); > DECLARE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT, > mem_alloc_profiling_key); > > +DECLARE_STATIC_KEY_FALSE(alloc_tag_trace_key); > + > static inline bool mem_alloc_profiling_enabled(void) > { > return > static_branch_maybe(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT, > &mem_alloc_profiling_key); > } > > +static inline bool alloc_tag_trace_enabled(void) > +{ > + return static_branch_unlikely(&alloc_tag_trace_key); > +} > + > +void alloc_tag_trace_mem_alloc(const void *ptr, struct alloc_tag *tag, > + size_t bytes); > + > +void alloc_tag_trace_mem_free(const void *ptr, struct alloc_tag *tag, > + size_t bytes); > + > +void __alloc_tag_trace_hit(struct alloc_tag *tag); > + > +static __always_inline void alloc_tag_trace_hit(struct alloc_tag *tag) > +{ > + if (alloc_tag_trace_enabled()) > + __alloc_tag_trace_hit(tag); > +} > + > bool mem_alloc_profiling_permanently_disabled(void); > > static inline struct alloc_tag_counters alloc_tag_read(struct alloc_tag *tag) > @@ -198,13 +219,19 @@ static inline bool alloc_tag_ref_set(union codetag_ref > *ref, struct alloc_tag *t > return true; > } > > -static inline void alloc_tag_add(union codetag_ref *ref, struct alloc_tag > *tag, size_t bytes) > +static inline void alloc_tag_add(union codetag_ref *ref, struct alloc_tag > *tag, size_t bytes, > + const void *ptr) > { > - if (likely(alloc_tag_ref_set(ref, tag))) > + if (likely(alloc_tag_ref_set(ref, tag))) { > this_cpu_add(tag->counters->bytes, bytes); > + > + if (alloc_tag_trace_enabled()) > + /* Trace successful allocs with their unique ptr */ > + alloc_tag_trace_mem_alloc(ptr, tag, bytes); > + } > } > > -static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes) > +static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes, const > void *ptr) > { > struct alloc_tag *tag; > > @@ -222,6 +249,10 @@ static inline void alloc_tag_sub(union codetag_ref *ref, > size_t bytes) > this_cpu_sub(tag->counters->bytes, bytes); > this_cpu_dec(tag->counters->calls); > > + if (alloc_tag_trace_enabled()) > + /* Trace frees with their unique ptr */ > + alloc_tag_trace_mem_free(ptr, tag, bytes); > + > ref->ct = NULL; > } > > @@ -243,25 +274,29 @@ static inline bool alloc_tag_is_inaccurate(struct > alloc_tag *tag) > static inline bool mem_alloc_profiling_enabled(void) { return false; } > static inline bool mem_alloc_profiling_permanently_disabled(void) { return > true; } > static inline void alloc_tag_add(union codetag_ref *ref, struct alloc_tag > *tag, > - size_t bytes) {} > -static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes) {} > + size_t bytes, const void *ptr) {} > +static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes, > + const void *ptr) {} > static inline void alloc_tag_set_inaccurate(struct alloc_tag *tag) {} > static inline bool alloc_tag_is_inaccurate(struct alloc_tag *tag) { return > false; } > +#define alloc_tag_trace_hit(_tag) /* NOOP */ > #define alloc_tag_record(p) do {} while (0) > > #endif /* CONFIG_MEM_ALLOC_PROFILING */ > > -#define alloc_hooks_tag(_tag, _do_alloc) \ > -({ \ > - typeof(_do_alloc) _res; \ > - if (mem_alloc_profiling_enabled()) { \ > - struct alloc_tag * __maybe_unused _old; \ > - _old = alloc_tag_save(_tag); \ > - _res = _do_alloc; \ > - alloc_tag_restore(_tag, _old); \ > - } else \ > - _res = _do_alloc; \ > - _res; \ > +#define alloc_hooks_tag(_tag, _do_alloc) > \ > +({ > \ > + typeof(_do_alloc) _res; > \ > + if (mem_alloc_profiling_enabled()) { > \ > + struct alloc_tag * __maybe_unused _old; > \ > + /* Fired here to cleanly capture the caller's stack trace */ > \ > + alloc_tag_trace_hit(_tag); > \ > + _old = alloc_tag_save(_tag); > \ > + _res = _do_alloc; > \ > + alloc_tag_restore(_tag, _old); > \ > + } else > \ > + _res = _do_alloc; > \ > + _res; > \ > }) > > #define alloc_hooks(_do_alloc) > \ > diff --git a/include/trace/events/alloc_tag.h > b/include/trace/events/alloc_tag.h > new file mode 100644 > index 000000000000..b79efb7dd25c > --- /dev/null > +++ b/include/trace/events/alloc_tag.h > @@ -0,0 +1,122 @@ > +/* SPDX-License-Identifier: GPL-2.0 */ > +#undef TRACE_SYSTEM > +#define TRACE_SYSTEM alloc_tag > + > +#if !defined(_TRACE_ALLOC_TAG_H) || defined(TRACE_HEADER_MULTI_READ) > +#define _TRACE_ALLOC_TAG_H > + > +#include <linux/tracepoint.h> > + > +/* > + * alloc_tag_hit is generated at the exact allocation call site and can be > + * used to capture a clean stack trace. > + * > + * To link this stack trace to the actual allocated memory chunk, tools must > + * correlate this event with the resulting alloc_tag_mem_alloced event. Since > + * multiple threads can hit the same tag simultaneously, tools must match > BOTH > + * the `tag` field and the implicitly recorded PID provided by the core > + * tracing subsystem. > + */ > +TRACE_EVENT(alloc_tag_hit, > + TP_PROTO(struct alloc_tag *tag), > + > + TP_ARGS(tag), > + > + TP_STRUCT__entry(__field(struct alloc_tag *, tag) > + __string(modname, tag->ct.modname ? tag->ct.modname : > "NONE") > + __string(filename, tag->ct.filename) > + __string(function, tag->ct.function) > + __field(unsigned int, lineno) > + ), > + > + TP_fast_assign(__entry->tag = tag; > + __assign_str(modname); > + __assign_str(filename); > + __assign_str(function); > + __entry->lineno = tag->ct.lineno; > + ), > + > + TP_printk("tag %p, module: %s, filename: %s, function %s, lineno %u", > + __entry->tag, > + __get_str(modname), > + __get_str(filename), > + __get_str(function), > + __entry->lineno > + ) > +); > + > +/* > + * alloc_tag_mem_alloced is generated after memory is successfully allocated. > + * It captures the exact byte size. > + * > + * The `ptr` value identifies the memory chunk for tracking its lifecycle > + * (e.g., matching it with alloc_tag_mem_freed). > + * - slab and percpu allocators: address of the object's codetag_ref > + * - page allocator: the head struct page of the allocation > + * > + * Because the kernel isolates active allocations within the task struct > + * (current->alloc_tag), this event will always share the same implicit PID > as > + * its corresponding alloc_tag_hit event. Tools should use the combination > + * PID + `tag` to correlate them. > + */ > +TRACE_EVENT(alloc_tag_mem_alloced, > + TP_PROTO(const void *ptr, struct alloc_tag *tag, size_t bytes), > + > + TP_ARGS(ptr, tag, bytes), > + > + TP_STRUCT__entry(__field(const void *, ptr) > + __field(struct alloc_tag *, tag) > + __field(size_t, bytes) > + ), > + > + TP_fast_assign(__entry->ptr = ptr; > + __entry->tag = tag; > + __entry->bytes = bytes; > + ), > + > + TP_printk("ptr %p, tag %p, bytes %zu", > + __entry->ptr, > + __entry->tag, > + __entry->bytes > + ) > +); > + > +/* > + * alloc_tag_mem_freed event is generated immediately before memory is > + * freed. The `ptr` value matches the one emitted during allocation, > + * allowing tools to match it to its corresponding allocation and > + * call stack. > + * > + * Page allocator caveat: Pages can be split or moved to another folio > + * after the alloc event with no new events generated for that. As a > + * result, a free event may carry a `ptr` that never appeared in an > + * alloc event, and the `bytes` may be smaller than in the matching > + * alloc event. Tools must not assume alloc bytes == free bytes, nor > + * that every free has a matching alloc. > + */ > +TRACE_EVENT(alloc_tag_mem_freed, > + TP_PROTO(const void *ptr, struct alloc_tag *tag, size_t bytes), > + > + TP_ARGS(ptr, tag, bytes), > + > + TP_STRUCT__entry(__field(const void *, ptr) > + __field(struct alloc_tag *, tag) > + __field(size_t, bytes) > + ), > + > + TP_fast_assign(__entry->ptr = ptr; > + __entry->tag = tag; > + __entry->bytes = bytes; > + ), > + > + TP_printk("ptr %p, tag %p, bytes %zu", > + __entry->ptr, > + __entry->tag, > + __entry->bytes > + ) > +); > + > +#endif /* _TRACE_ALLOC_TAG_H */ > + > +/* This part must be outside protection */ > +#include <trace/define_trace.h> > diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c > index 82e2c3448dcf..ca2412a67312 100644 > --- a/mm/alloc_tag.c > +++ b/mm/alloc_tag.c > @@ -18,6 +18,9 @@ > #include <linux/kmemleak.h> > #include <uapi/linux/alloc_tag.h> > > +#define CREATE_TRACE_POINTS > +#include <trace/events/alloc_tag.h> > + > #include "internal.h" > #include "page_alloc.h" > > @@ -54,6 +57,9 @@ EXPORT_SYMBOL(mem_alloc_profiling_key); > > DEFINE_STATIC_KEY_FALSE(mem_profiling_compressed); > > +DEFINE_STATIC_KEY_FALSE(alloc_tag_trace_key); > +EXPORT_SYMBOL(alloc_tag_trace_key); > + > struct alloc_tag_kernel_section kernel_tags = { NULL, 0 }; > unsigned long alloc_tag_ref_mask; > int alloc_tag_ref_offs; > @@ -484,6 +490,28 @@ static const struct proc_ops allocinfo_proc_ops = { > #endif > }; > > +void __alloc_tag_trace_hit(struct alloc_tag *tag) > +{ > + if (unlikely(!tag)) > + return; > + trace_alloc_tag_hit(tag); > +} > +EXPORT_SYMBOL(__alloc_tag_trace_hit); > + > +void alloc_tag_trace_mem_alloc(const void *ptr, struct alloc_tag *tag, > + size_t bytes) > +{ > + trace_alloc_tag_mem_alloced(ptr, tag, bytes); > +} > +EXPORT_SYMBOL(alloc_tag_trace_mem_alloc); > + > +void alloc_tag_trace_mem_free(const void *ptr, struct alloc_tag *tag, > + size_t bytes) > +{ > + trace_alloc_tag_mem_freed(ptr, tag, bytes); > +} > +EXPORT_SYMBOL(alloc_tag_trace_mem_free); > + > size_t alloc_tag_top_users(struct codetag_bytes *tags, size_t count, bool > can_sleep) > { > struct codetag_iterator iter; > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 7682aecc2c07..5e3e411d4ffe 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -1239,7 +1239,7 @@ void __pgalloc_tag_add(struct page *page, struct > task_struct *task, > union codetag_ref ref; > > if (likely(get_page_tag_ref(page, &ref, &handle))) { > - alloc_tag_add(&ref, task->alloc_tag, PAGE_SIZE * nr); > + alloc_tag_add(&ref, task->alloc_tag, PAGE_SIZE * nr, page); > update_page_tag_ref(handle, &ref); > put_page_tag_ref(handle); > } else { > @@ -1268,7 +1268,7 @@ void __pgalloc_tag_sub(struct page *page, unsigned int > nr) > union codetag_ref ref; > > if (get_page_tag_ref(page, &ref, &handle)) { > - alloc_tag_sub(&ref, PAGE_SIZE * nr); > + alloc_tag_sub(&ref, PAGE_SIZE * nr, page); > update_page_tag_ref(handle, &ref); > put_page_tag_ref(handle); > } > diff --git a/mm/percpu.c b/mm/percpu.c > index 3eff382e565a..12b6c97d1596 100644 > --- a/mm/percpu.c > +++ b/mm/percpu.c > @@ -1695,15 +1695,19 @@ static void pcpu_alloc_tag_alloc_hook(struct > pcpu_chunk *chunk, int off, > size_t size) > { > if (mem_alloc_profiling_enabled() && likely(chunk->obj_exts)) { > - alloc_tag_add(&chunk->obj_exts[off >> PCPU_MIN_ALLOC_SHIFT].tag, > - current->alloc_tag, size); > + union codetag_ref *ref = &chunk->obj_exts[off >> > PCPU_MIN_ALLOC_SHIFT].tag; > + > + alloc_tag_add(ref, current->alloc_tag, size, ref); > } > } > > static void pcpu_alloc_tag_free_hook(struct pcpu_chunk *chunk, int off, > size_t size) > { > - if (mem_alloc_profiling_enabled() && likely(chunk->obj_exts)) > - alloc_tag_sub(&chunk->obj_exts[off >> > PCPU_MIN_ALLOC_SHIFT].tag, size); > + if (mem_alloc_profiling_enabled() && likely(chunk->obj_exts)) { > + union codetag_ref *ref = &chunk->obj_exts[off >> > PCPU_MIN_ALLOC_SHIFT].tag; > + > + alloc_tag_sub(ref, size, ref); > + } > } > #else > static void pcpu_alloc_tag_alloc_hook(struct pcpu_chunk *chunk, int off, > diff --git a/mm/slub.c b/mm/slub.c > index 544cff39762c..a866576ccb98 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2404,7 +2404,7 @@ __alloc_tagging_slab_alloc_hook(struct kmem_cache *s, > void *object, gfp_t flags, > obj_ext = slab_obj_ext(s, slab, obj_exts, object); > ref = slab_obj_ext_codetag_ref(slab, obj_ext); > > - alloc_tag_add(ref, current->alloc_tag, s->size); > + alloc_tag_add(ref, current->alloc_tag, s->size, ref); > > put_slab_obj_exts(obj_exts); > } else { > @@ -2444,9 +2444,11 @@ __alloc_tagging_slab_free_hook(struct kmem_cache *s, > struct slab *slab, void **p > get_slab_obj_exts(obj_exts); > for (int i = 0; i < objects; i++) { > struct slabobj_ext *ext; > + union codetag_ref *ref; > > ext = slab_obj_ext(s, slab, obj_exts, p[i]); > - alloc_tag_sub(slab_obj_ext_codetag_ref(slab, ext), s->size); > + ref = slab_obj_ext_codetag_ref(slab, ext); > + alloc_tag_sub(ref, s->size, ref); > } > put_slab_obj_exts(obj_exts); > }
