On Thu, Sep 24, 2026 at 1:16 AM Hao Ge <[email protected]> wrote:
>
> Hi Abhishek
>
> On 2026/9/22 05:26, Abhishek Bapat wrote:
> > Introduce a new IOCTL (`ALLOCINFO_IOC_TOGGLE_TRACE`) to selectively
> > toggle tracing on exact allocation call sites. Userspace tools can
> > use the existing filtering mechanism to specify the set of tags to
> > toggle tracing for.
> >
> > To facilitate low overhead execution for non-targeted call sites, add a
> > new `CODETAG_FLAG_TRACE_ON` flag to `struct codetag` to track per-site
> > activation. Protect these conditional branch evaluations using a global
> > `alloc_tag_trace_key` static branch and an inline static key check
> > pattern in the allocator hooks (`alloc_tag_add`, `alloc_tag_sub`, etc).
> > This ensures that the trace events are entirely skipped when no
> > allocation call sites are being actively traced, leaving only a NOP on
> > the allocation fast path.
> >
> > Signed-off-by: Abhishek Bapat <[email protected]>
> > ---
> > include/linux/alloc_tag.h | 39 ++++++++----
> > include/linux/codetag.h | 5 +-
> > include/uapi/linux/alloc_tag.h | 9 +++
> > mm/alloc_tag.c | 110 ++++++++++++++++++++++++++++++++-
> > 4 files changed, 147 insertions(+), 16 deletions(-)
> >
> > diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
> > index 2994934cf44a..dc86f8997476 100644
> > --- a/include/linux/alloc_tag.h
> > +++ b/include/linux/alloc_tag.h
> > @@ -136,9 +136,36 @@ static inline bool mem_alloc_profiling_enabled(void)
> > &mem_alloc_profiling_key);
> > }
> >
> > +static inline void alloc_tag_set_inaccurate(struct alloc_tag *tag)
> > +{
> > + atomic_or(CODETAG_FLAG_INACCURATE, &tag->ct.flags);
> > +}
> > +
> > +static inline bool alloc_tag_is_inaccurate(struct alloc_tag *tag)
> > +{
> > + return !!(atomic_read(&tag->ct.flags) & CODETAG_FLAG_INACCURATE);
> > +}
> > +
> > +static inline void alloc_tag_set_traced(struct alloc_tag *tag)
> > +{
> > + atomic_or(CODETAG_FLAG_TRACE_ON, &tag->ct.flags);
> > +}
> > +
> > +static inline void alloc_tag_clear_traced(struct alloc_tag *tag)
> > +{
> > + atomic_andnot(CODETAG_FLAG_TRACE_ON, &tag->ct.flags);
> > +}
> > +
> > +static inline bool alloc_tag_is_traced(const struct alloc_tag *tag)
> > +{
> > + return !!(atomic_read(&tag->ct.flags) & CODETAG_FLAG_TRACE_ON);
> > +}
> > +
> > static inline bool alloc_tag_trace_enabled(const struct alloc_tag *tag)
> > {
> > - return static_branch_unlikely(&alloc_tag_trace_key);
> > + if (static_branch_unlikely(&alloc_tag_trace_key))
> > + return tag && alloc_tag_is_traced(tag);
> > + return false;
> > }
> >
> > void alloc_tag_trace_mem_alloc(union codetag_ref *ref, struct alloc_tag
> > *tag,
> > @@ -255,16 +282,6 @@ static inline void alloc_tag_sub(union codetag_ref
> > *ref, size_t bytes)
> > ref->ct = NULL;
> > }
> >
> > -static inline void alloc_tag_set_inaccurate(struct alloc_tag *tag)
> > -{
> > - tag->ct.flags |= CODETAG_FLAG_INACCURATE;
> > -}
> > -
> > -static inline bool alloc_tag_is_inaccurate(struct alloc_tag *tag)
> > -{
> > - return !!(tag->ct.flags & CODETAG_FLAG_INACCURATE);
> > -}
> > -
> > #define alloc_tag_record(p) ((p) = current->alloc_tag)
> >
> > #else /* CONFIG_MEM_ALLOC_PROFILING */
> > diff --git a/include/linux/codetag.h b/include/linux/codetag.h
> > index a25a085c2df1..f728295d50c0 100644
> > --- a/include/linux/codetag.h
> > +++ b/include/linux/codetag.h
> > @@ -18,6 +18,7 @@ struct module;
> >
> > /* codetag flags */
> > #define CODETAG_FLAG_INACCURATE (1 << 0)
> > +#define CODETAG_FLAG_TRACE_ON (1 << 1)
> >
> > /*
> > * An instance of this structure is created in a special ELF section at
> > every
> > @@ -25,7 +26,7 @@ struct module;
> > * an array of these.
> > */
> > struct codetag {
> > - unsigned int flags;
> > + atomic_t flags;
> > unsigned int lineno;
> > const char *modname;
> > const char *function;
> > @@ -71,7 +72,7 @@ struct codetag_iterator {
> > .function = __func__, \
> > .filename = __FILE__, \
> > .lineno = __LINE__, \
> > - .flags = 0, \
> > + .flags = ATOMIC_INIT(0), \
> > }
> >
> > void codetag_lock_module_list(struct codetag_type *cttype);
> > diff --git a/include/uapi/linux/alloc_tag.h b/include/uapi/linux/alloc_tag.h
> > index 7d4618bea043..069ab8341e87 100644
> > --- a/include/uapi/linux/alloc_tag.h
> > +++ b/include/uapi/linux/alloc_tag.h
> > @@ -85,9 +85,16 @@ struct allocinfo_get_at {
> > struct allocinfo_tag_data data;
> > };
> >
> > +struct allocinfo_toggle_traces {
> > + /* inputs */
> > + struct allocinfo_tag fields;
> > + __u64 enable;
> > +};
> > +
> > #define _ALLOCINFO_IOC_CONTENT_ID 0
> > #define _ALLOCINFO_IOC_GET_AT 1
> > #define _ALLOCINFO_IOC_GET_NEXT 2
> > +#define _ALLOCINFO_IOC_TOGGLE_TRACE 3
> >
> > #define ALLOCINFO_IOC_BASE 0xA6
> > #define ALLOCINFO_IOC_CONTENT_ID _IOR(ALLOCINFO_IOC_BASE,
> > _ALLOCINFO_IOC_CONTENT_ID, \
> > @@ -96,5 +103,7 @@ struct allocinfo_get_at {
> > struct allocinfo_get_at)
> > #define ALLOCINFO_IOC_GET_NEXT _IOR(ALLOCINFO_IOC_BASE,
> > _ALLOCINFO_IOC_GET_NEXT, \
> > struct allocinfo_tag_data)
> > +#define ALLOCINFO_IOC_TOGGLE_TRACE _IOW(ALLOCINFO_IOC_BASE,
> > _ALLOCINFO_IOC_TOGGLE_TRACE, \
> > + struct allocinfo_toggle_traces)
> >
> > #endif /* _UAPI_ALLOC_TAG_H */
> > diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
> > index a5339767efd5..fb179321a2a1 100644
> > --- a/mm/alloc_tag.c
> > +++ b/mm/alloc_tag.c
> > @@ -61,6 +61,15 @@ DEFINE_STATIC_KEY_FALSE(mem_profiling_compressed);
> > DEFINE_STATIC_KEY_FALSE(alloc_tag_trace_key);
> > EXPORT_SYMBOL(alloc_tag_trace_key);
> >
> > +static atomic_t alloc_tag_trace_cnt = ATOMIC_INIT(0);
> > +
> > +/*
> > + * As `codetag_lock_module_list` is a read lock, we need an additional
> > mutex
> > + * to protect against the race conditions involved in the alloc tag trace
> > + * toggle path.
> > + */
> > +static DEFINE_MUTEX(alloc_tag_trace_mutex);
> > +
> > struct alloc_tag_kernel_section kernel_tags = { NULL, 0 };
> > unsigned long alloc_tag_ref_mask;
> > int alloc_tag_ref_offs;
> > @@ -297,7 +306,7 @@ static bool matches_filter(struct codetag *ct, struct
> > allocinfo_filter *filter,
> > return false;
> >
> > if (filter->mask & ALLOCINFO_FILTER_MASK_INACCURATE) {
> > - inaccurate = !!(ct->flags & CODETAG_FLAG_INACCURATE);
> > + inaccurate = alloc_tag_is_inaccurate(ct_to_alloc_tag(ct));
> > if (inaccurate != !!(filter->inaccurate))
> > return false;
> > }
> > @@ -444,6 +453,81 @@ static int allocinfo_ioctl_get_next(struct seq_file
> > *m, void __user *arg)
> > return ret;
> > }
> >
> > +static bool alloc_tag_trace_toggle(struct alloc_tag *tag, bool enable)
> > +{
> > + if (enable) {
> > + if (alloc_tag_is_traced(tag))
> > + return false;
> > +
> > + alloc_tag_set_traced(tag);
> > + if (atomic_fetch_inc(&alloc_tag_trace_cnt) == 0)
> > + static_branch_enable(&alloc_tag_trace_key);
> > + } else {
> > + if (!alloc_tag_is_traced(tag))
> > + return false;
> > +
> > + alloc_tag_clear_traced(tag);
> > + if (atomic_dec_and_test(&alloc_tag_trace_cnt))
> > + static_branch_disable(&alloc_tag_trace_key);
> > + }
> > +
> > + return true;
> > +}
> > +
> > +/*
> > + * Toggles context capture for a specified allocation.
> > + */
> > +static int allocinfo_ioctl_toggle_trace(struct seq_file *m, void __user
> > *arg)
> > +{
> > + struct allocinfo_toggle_traces params;
> > + struct codetag_iterator iter;
> > + struct codetag *ct;
> > + int matches = 0, successes = 0, ret;
> > +
> > + if (!capable(CAP_SYS_ADMIN))
> > + return -EPERM;
> > +
> > + if (copy_from_user(¶ms, arg, sizeof(params)))
> > + return -EFAULT;
> > +
> > + codetag_lock_module_list(alloc_tag_cttype);
> > +
> > + struct allocinfo_filter filter = {
> > + .mask = ALLOCINFO_FILTER_MASK_MODNAME |
> > + ALLOCINFO_FILTER_MASK_FUNCTION |
> > + ALLOCINFO_FILTER_MASK_FILENAME |
> > + ALLOCINFO_FILTER_MASK_LINENO,
> > + .fields = params.fields,
> > + };
> > +
> > + iter = codetag_get_ct_iter(alloc_tag_cttype);
> > +
> > + /* Toggle tracing on all codetags that match */
> > + while ((ct = codetag_next_ct(&iter))) {
> > + if (matches_filter(ct, &filter, NULL, NULL)) {
> > + matches++;
> > +
> > + mutex_lock(&alloc_tag_trace_mutex);
> > + if (alloc_tag_trace_toggle(ct_to_alloc_tag(ct),
> > !!params.enable))
> > + successes++;
> > + mutex_unlock(&alloc_tag_trace_mutex);
> > + }
> > + }
> > +
> > + if (matches == 0)
> > + /* Nothing matched the filter */
> > + ret = -ENOENT;
> > + else if (successes == 0)
> > + /* Items matched, but were already in the requested state */
> > + ret = -EINVAL;
>
> Why do we return -EINVAL in this path?
> -EINVAL can mislead users into thinking their arguments are wrong.
> That's not the case; the trace request they want is already active.
> Maybe -EEXIST ?
Or even 0 since the configuration already matches what the user requested.
>
> Thanks
> Best Regards
> Hao
>
> > + else
> > + ret = 0;
> > +
> > + codetag_unlock_module_list(alloc_tag_cttype);
> > +
> > + return ret;
> > +}
> > +
> > /*
> > * Entry point ioctl function for /proc/allocinfo routing requests to
> > fetch the
> > * layout content ID, seek to a specific tag, or read sequential tags.
> > @@ -464,6 +548,9 @@ static long allocinfo_ioctl(struct file *file, unsigned
> > int cmd,
> > case ALLOCINFO_IOC_GET_NEXT:
> > ret = allocinfo_ioctl_get_next(file->private_data, arg);
> > break;
> > + case ALLOCINFO_IOC_TOGGLE_TRACE:
> > + ret = allocinfo_ioctl_toggle_trace(file->private_data, arg);
> > + break;
> > default:
> > ret = -ENOIOCTLCMD;
> > break;
> > @@ -493,8 +580,6 @@ static const struct proc_ops allocinfo_proc_ops = {
> >
> > void __alloc_tag_trace_hit(struct alloc_tag *tag)
> > {
> > - if (unlikely(!tag))
> > - return;
> > trace_alloc_tag_hit(tag);
> > }
> > EXPORT_SYMBOL(__alloc_tag_trace_hit);
> > @@ -1043,6 +1128,24 @@ static int load_module(struct module *mod, struct
> > codetag *start, struct codetag
> > return 0;
> > }
> >
> > +static void unload_module(struct module *mod, struct codetag *start,
> > struct codetag *stop)
> > +{
> > + struct alloc_tag *start_tag = ct_to_alloc_tag(start);
> > + struct alloc_tag *stop_tag = ct_to_alloc_tag(stop);
> > + struct alloc_tag *tag;
> > +
> > + /*
> > + * Turn tracing off for the tags of the module being unloaded. Without
> > + * this, `alloc_tag_trace_cnt` would never reach zero and tracing
> > would
> > + * stay enabled forever.
> > + *
> > + * `alloc_tag_trace_mutex` is not needed here as this code path is
> > + * protected by a `down_write(&cttype->mod_lock)`.
> > + */
> > + for (tag = start_tag; tag < stop_tag; tag++)
> > + alloc_tag_trace_toggle(tag, false);
> > +}
> > +
> > static void replace_module(struct module *mod, struct module *new_mod)
> > {
> > MA_STATE(mas, &mod_area_mt, 0, module_tags.size);
> > @@ -1369,6 +1472,7 @@ static int __init alloc_tag_init(void)
> > .alloc_section_mem = reserve_module_tags,
> > .free_section_mem = release_module_tags,
> > .module_load = load_module,
> > + .module_unload = unload_module,
> > .module_replaced = replace_module,
> > #endif
> > };
