Re: [PATCH v2 1/4] alloc_tag: Add trace events for tracing allocations

[Suren Baghdasaryan]

On Tue, Oct 13, 2026 at 10:12 AM Hao Ge <hao.ge@xxxxxxxxx> wrote:
>
> 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 <abhishekbapat@xxxxxxxxxx>
>
> Acked-by: Hao Ge <hao.ge@xxxxxxxxx>

Acked-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

>
> > ---
> >  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))

nit: Is there a case when we pass tag=NULL here?

> > +             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);
> >  }