Re: [PATCH 1/4] alloc_tag: Add trace events for tracing allocations

From: Suren Baghdasaryan

Date: Tue Sep 29 2026 - 22:23:40 EST

On Tue, Sep 29, 2026 at 5:32 PM Abhishek Bapat <abhishekbapat@xxxxxxxxxx> wrote:
>
> On Sun, Sep 27, 2026 at 7:10 PM Hao Ge <hao.ge@xxxxxxxxx> wrote:
> >
> > Hi Suren
> >
> > On 2026/9/25 14:14, Suren Baghdasaryan wrote:
> > > On Thu, Sep 24, 2026 at 12:21 AM Hao Ge <hao.ge@xxxxxxxxx> wrote:
> > >>
> > >> Hi Abhishek
> > >>
> > >> Sorry, I merged and tested your patches today and found a few details,
> > >> so I've started a separate mail thread for this review.
> > >>
> > >> On 2026/9/22 05:26, 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 codetag_ref metadata pointer.
> > >>> 3. `alloc_tag_mem_freed`: Fired in alloc_tag_sub right before memory is
> > >>> freed, yielding the same codetag_ref 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 `codetag_ref`
> > >>> 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 `codetag_ref`. The tool uses this reference to look up the
> > >>> original allocation record, correlates the free, and safely retires
> > >>> the tracking entry.
> > >>>
> > >>> 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.
> > >>> Nore that the mechanism to enabl tag tracing is implemented in the next
> > >>> patch, therefore for now, `alloc_tag_trace_key` stays always unset.
> > >>>
> > >>> Signed-off-by: Abhishek Bapat <abhishekbapat@xxxxxxxxxx>
> > >>> ---
> > >>> MAINTAINERS | 1 +
> > >>> include/linux/alloc_tag.h | 57 ++++++++++++---
> > >>> include/trace/events/alloc_tag.h | 122 +++++++++++++++++++++++++++++++
> > >>> mm/alloc_tag.c | 28 +++++++
> > >>> 4 files changed, 196 insertions(+), 12 deletions(-)
> > >>> create mode 100644 include/trace/events/alloc_tag.h
> > >>>
> > >>> diff --git a/MAINTAINERS b/MAINTAINERS
> > >>> index 24420a8c06d0..29e1f7915cb9 100644
> > >>> --- a/MAINTAINERS
> > >>> +++ b/MAINTAINERS
> > >>> @@ -17096,6 +17096,7 @@ 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..2994934cf44a 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(const struct alloc_tag *tag)
> > >>> +{
> > >>> + return static_branch_unlikely(&alloc_tag_trace_key);
> > >>> +}
> > >>> +
> > >>> +void alloc_tag_trace_mem_alloc(union codetag_ref *ref, struct alloc_tag *tag,
> > >>> + size_t bytes);
> > >>> +
> > >>> +void alloc_tag_trace_mem_free(union codetag_ref *ref, struct alloc_tag *tag,
> > >>> + size_t bytes);
> > >>> +
> > >>> +void __alloc_tag_trace_hit(struct alloc_tag *tag);
> > >>> +
> > >>> +static inline void alloc_tag_trace_hit(struct alloc_tag *tag)
> > >>> +{
> > >>> + if (alloc_tag_trace_enabled(tag))
> > >>> + __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)
> > >>> @@ -200,8 +221,13 @@ static inline bool alloc_tag_ref_set(union codetag_ref *ref, struct alloc_tag *t
> > >>>
> > >>> static inline void alloc_tag_add(union codetag_ref *ref, struct alloc_tag *tag, size_t bytes)
> > >>> {
> > >>> - 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(tag))
> > >>> + /* Trace successful allocs with their unique ref */
> > >>> + alloc_tag_trace_mem_alloc(ref, tag, bytes);
> > >>> + }
> > >>> }
> > >>>
> > >>> static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes)
> > >>> @@ -222,6 +248,10 @@ 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(tag))
> > >>> + /* Trace frees with their unique ref */
> > >>> + alloc_tag_trace_mem_free(ref, tag, bytes);
> > >>> +
> > >>> ref->ct = NULL;
> > >>> }
> > >>>
> > >>> @@ -247,21 +277,24 @@ static inline void alloc_tag_add(union codetag_ref *ref, struct alloc_tag *tag,
> > >>> static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes) {}
> > >>> 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 */
> > >>
> > >> I'd prefer we use `do {} while (0)` here, following the same pattern
> > >> as alloc_tag_record.
> > >>
> > >>> #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..af2182501864
> > >>> --- /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
> > >>> + )
> > >>> +);
> > >>>
> > >>
> > >> When I trace an interface provided by a kernel build-in such as shmem,
> > >> this is what I see in the trace:
> > >>
> > >> trig-6838 [004] ..... 1309.321906: alloc_tag_hit: tag 000000007874a2c9, module: NONE, filename: mm/shmem.c, function shmem_alloc_folio, lineno 2138
> > >>
> > >> The module field shows NONE, which looks a bit odd.
> > >>
> > >>> +/*
> > >>> + * alloc_tag_mem_alloced is generated after memory is successfully allocated.
> > >>> + * It captures the exact byte size.
> > >>> + *
> > >>> + * The `ref` pointer identifies the memory chunk for tracking its lifecycle
> > >>> + * (e.g., matching it with alloc_tag_mem_freed).
> > >>> + *
> > >>> + * Because the kernel isolates active allocations within the task struct
> > >>> + * (current->alloc_tag), this even 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(union codetag_ref *ref, struct alloc_tag *tag, size_t bytes),
> > >>> +
> > >>> + TP_ARGS(ref, tag, bytes),
> > >>> +
> > >>> + TP_STRUCT__entry(
> > >>> + __field(union codetag_ref *, ref)
> > >>> + __field(struct alloc_tag *, tag)
> > >>> + __field(size_t, bytes)
> > >>> + ),
> > >>> +
> > >>> + TP_fast_assign(
> > >>> + __entry->ref = ref;
> > >>> + __entry->tag = tag;
> > >>> + __entry->bytes = bytes;
> > >>> + ),
> > >>> +
> > >>> + TP_printk("reference %p, tag %p, bytes %zu",
> > >>> + __entry->ref,
> > >>> + __entry->tag,
> > >>> + __entry->bytes
> > >>> + )
> > >>> +);
> > >>> +
> > >>> +/*
> > >>> + * alloc_tag_mem_freed event is generated immediately before memory is
> > >>> + * freed. The `ref` pointer matches the one emitted during allocation,
> > >>> + * allowing tools to match it to it's corresponding allocation and
> > >>> + * call stack.
> > >>> + */
> > >>
> > >> But the thread for allocation and free may not be the same.
> > >> For example, memory allocated in thread A could be freed by kswapd.
> > >> In this case, would the ref fail to match and falsely report a memory leak?
> > >
> > > No, ref is unique within the system, so even if alloc and free happen
> > > in different threads it still can be used to match them. The
> > > thread+tag is used only when matching alloc_tag_hit with
> > > alloc_tag_mem_alloced because at the time of alloc_tag_hit we don't
> > > yet have the ref.
> > >
> >
> > "The ref is unique within the system" is true of the obj_ext/page_ext slot,
> > but for page allocations the event does not carry the slot's address,
> > it carries the address of a stack temporary.
> >
> > /* Should be called only if mem_alloc_profiling_enabled() */
> > static noinline
> > void __pgalloc_tag_add(struct page *page, struct task_struct *task,
> > unsigned int nr, unsigned int alloc_flags)
> > {
> > union pgtag_ref_handle handle;
> > union codetag_ref ref; <- stack temporary
> >
> > if (likely(get_page_tag_ref(page, &ref, &handle))) {
> > alloc_tag_add(&ref, task->alloc_tag, PAGE_SIZE * nr);
> > update_page_tag_ref(handle, &ref);
> > put_page_tag_ref(handle);
> > } else {
> > ......
> > }
> > }
> >
> > /* Should be called only if mem_alloc_profiling_enabled() */
> > static noinline
> > void __pgalloc_tag_sub(struct page *page, unsigned int nr)
> > {
> > union pgtag_ref_handle handle;
> > union codetag_ref ref; <- stack temporary
> >
> > if (get_page_tag_ref(page, &ref, &handle)) {
> > alloc_tag_sub(&ref, PAGE_SIZE * nr);
> > update_page_tag_ref(handle, &ref);
> > put_page_tag_ref(handle);
> > }
> > }
> >
> > Or am I missing / confusing something here?
> >
>
> This is a legitimate concern, I went through the code again, the
> codetag_ref pointer approach works for percpu and slab allocators but
> don't work for the page allocator.

Ugh! You are absolutely correct. I'm still thinking in terms of
pre-alloctag compression, before we started storing codetag references
in the page flags.

>
> Suren, Hao, I could potentially pass in the struct page * to
> __alloc_tag_add and __alloc_tag_sub from the page allocator and keep
> the codepath for the percpu and slab allocators the same. What do you
> think? I think that should solve this concern.

Yes, that should solve this ambiguity and I think it would be the
simplest approach here.

>
> > Thanks
> > Best Regards
> > Hao
> >
> > >>
> > >> Thanks
> > >> Best Regards
> > >> Hao
> > >>
> > >>> +TRACE_EVENT(alloc_tag_mem_freed,
> > >>> +
> > >>> + TP_PROTO(union codetag_ref *ref, struct alloc_tag *tag, size_t bytes),
> > >>> +
> > >>> + TP_ARGS(ref, tag, bytes),
> > >>> +
> > >>> + TP_STRUCT__entry(
> > >>> + __field(union codetag_ref *, ref)
> > >>> + __field(struct alloc_tag *, tag)
> > >>> + __field(size_t, bytes)
> > >>> + ),
> > >>> +
> > >>> + TP_fast_assign(
> > >>> + __entry->ref = ref;
> > >>> + __entry->tag = tag;
> > >>> + __entry->bytes = bytes;
> > >>> + ),
> > >>> +
> > >>> + TP_printk("reference %p, tag %p, bytes %zu",
> > >>> + __entry->ref,
> > >>> + __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 f30ef8dd24c7..a5339767efd5 100644
> > >>> --- a/mm/alloc_tag.c
> > >>> +++ b/mm/alloc_tag.c
> > >>> @@ -19,6 +19,9 @@
> > >>> #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"
> > >>>
> > >>> @@ -55,6 +58,9 @@ 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;
> > >>> @@ -485,6 +491,28 @@ 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(union codetag_ref *ref, struct alloc_tag *tag,
> > >>> + size_t bytes)
> > >>> +{
> > >>> + trace_alloc_tag_mem_alloced(ref, tag, bytes);
> > >>> +}
> > >>> +EXPORT_SYMBOL(alloc_tag_trace_mem_alloc);
> > >>> +
> > >>> +void alloc_tag_trace_mem_free(union codetag_ref *ref, struct alloc_tag *tag,
> > >>> + size_t bytes)
> > >>> +{
> > >>> + trace_alloc_tag_mem_freed(ref, 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;