Re: [PATCH v2] mm/alloc_tag: replace fixed-size early PFN array with dynamic linked list

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Apr 20, 2026 at 8:15 PM Hao Ge <hao.ge@linux.dev> wrote:
>
> Pages allocated before page_ext is available have their codetag left
> uninitialized. Track these early PFNs and clear their codetag in
> clear_early_alloc_pfn_tag_refs() to avoid "alloc_tag was not set"
> warnings when they are freed later.
>
> Currently a fixed-size array of 8192 entries is used, with a warning if
> the limit is exceeded. However, the number of early allocations depends
> on the number of CPUs and can be larger than 8192.
>
> Replace the fixed-size array with a dynamically allocated linked list.
> Each page is carved into early_pfn_node entries and the remainder is
> kept as a freelist for subsequent allocations.
>
> The list nodes themselves are allocated via alloc_page(), which would
> trigger __pgalloc_tag_add() -> alloc_tag_add_early_pfn() ->
> alloc_early_pfn_node() and recurse indefinitely.  Introduce
> __GFP_NO_CODETAG (reuses the %__GFP_NO_OBJ_EXT bit) and pass
> gfp_flags through pgalloc_tag_add() so that the early path can skip
> recording allocations that carry this flag.

Hi Hao,
Thanks for following up on this!

>
> Signed-off-by: Hao Ge <hao.ge@linux.dev>
> ---
> v2:
> - Use cmpxchg to atomically update early_pfn_pages, preventing page leak under concurrent allocation
> - Pass gfp_flags through the full call chain and use gfpflags_allow_blocking()
>   to select GFP_KERNEL vs GFP_ATOMIC, avoiding unnecessary GFP_ATOMIC in process context
> ---
>  include/linux/alloc_tag.h |  22 +++++++-
>  lib/alloc_tag.c           | 102 ++++++++++++++++++++++++++------------
>  mm/page_alloc.c           |  29 +++++++----
>  3 files changed, 108 insertions(+), 45 deletions(-)
>
> diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
> index 02de2ede560f..2fa695bd3c53 100644
> --- a/include/linux/alloc_tag.h
> +++ b/include/linux/alloc_tag.h
> @@ -150,6 +150,23 @@ static inline struct alloc_tag_counters alloc_tag_read(struct alloc_tag *tag)
>  }
>
>  #ifdef CONFIG_MEM_ALLOC_PROFILING_DEBUG
> +/*
> + * Skip early PFN recording for a page allocation.  Reuses the
> + * %__GFP_NO_OBJ_EXT bit.  Used by alloc_early_pfn_node() to avoid
> + * recursion when allocating pages for the early PFN tracking list
> + * itself.
> + *
> + * Callers must set the codetag to CODETAG_EMPTY (via
> + * clear_page_tag_ref()) before freeing pages allocated with this
> + * flag once page_ext becomes available, otherwise
> + * alloc_tag_sub_check() will trigger a warning.
> + */
> +#define __GFP_NO_CODETAG       __GFP_NO_OBJ_EXT
> +
> +static inline bool should_record_early_pfn(gfp_t gfp_flags)
> +{
> +       return !(gfp_flags & __GFP_NO_CODETAG);
> +}
>  static inline void alloc_tag_add_check(union codetag_ref *ref, struct alloc_tag *tag)
>  {
>         WARN_ONCE(ref && ref->ct && !is_codetag_empty(ref),
> @@ -163,11 +180,12 @@ static inline void alloc_tag_sub_check(union codetag_ref *ref)
>  {
>         WARN_ONCE(ref && !ref->ct, "alloc_tag was not set\n");
>  }
> -void alloc_tag_add_early_pfn(unsigned long pfn);
> +void alloc_tag_add_early_pfn(unsigned long pfn, gfp_t gfp_flags);
>  #else
>  static inline void alloc_tag_add_check(union codetag_ref *ref, struct alloc_tag *tag) {}
>  static inline void alloc_tag_sub_check(union codetag_ref *ref) {}
> -static inline void alloc_tag_add_early_pfn(unsigned long pfn) {}
> +static inline void alloc_tag_add_early_pfn(unsigned long pfn, gfp_t gfp_flags) {}
> +static inline bool should_record_early_pfn(gfp_t gfp_flags) { return true; }


If CONFIG_MEM_ALLOC_PROFILING_DEBUG=n why should we record early pfns?

>  #endif
>
>  /* Caller should verify both ref and tag to be valid */
> diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> index ed1bdcf1f8ab..cfc68e397eba 100644
> --- a/lib/alloc_tag.c
> +++ b/lib/alloc_tag.c
> @@ -766,45 +766,75 @@ static __init bool need_page_alloc_tagging(void)
>   * Some pages are allocated before page_ext becomes available, leaving
>   * their codetag uninitialized. Track these early PFNs so we can clear
>   * their codetag refs later to avoid warnings when they are freed.
> - *
> - * Early allocations include:
> - *   - Base allocations independent of CPU count
> - *   - Per-CPU allocations (e.g., CPU hotplug callbacks during smp_init,
> - *     such as trace ring buffers, scheduler per-cpu data)
> - *
> - * For simplicity, we fix the size to 8192.
> - * If insufficient, a warning will be triggered to alert the user.
> - *
> - * TODO: Replace fixed-size array with dynamic allocation using
> - * a GFP flag similar to ___GFP_NO_OBJ_EXT to avoid recursion.
>   */
> -#define EARLY_ALLOC_PFN_MAX            8192
> +struct early_pfn_node {
> +       struct early_pfn_node   *next;
> +       unsigned long           pfn;
> +};
> +
> +#define NODES_PER_PAGE         (PAGE_SIZE / sizeof(struct early_pfn_node))
>
> -static unsigned long early_pfns[EARLY_ALLOC_PFN_MAX] __initdata;
> -static atomic_t early_pfn_count __initdata = ATOMIC_INIT(0);
> +static struct early_pfn_node *early_pfn_list __initdata;
> +static struct early_pfn_node *early_pfn_freelist __initdata;
> +static struct page *early_pfn_pages __initdata;

This early_pfn_node linked list seems overly complex. Why not just
allocate a page and use page->lru to place it into a linked list? I
think the code will end up much simpler.

>
> -static void __init __alloc_tag_add_early_pfn(unsigned long pfn)
> +static struct early_pfn_node *__init alloc_early_pfn_node(gfp_t gfp_flags)
>  {
> -       int old_idx, new_idx;
> +       struct early_pfn_node *ep, *new;
> +       struct page *page, *old_page;
> +       gfp_t gfp = gfpflags_allow_blocking(gfp_flags) ? GFP_KERNEL : GFP_ATOMIC;
> +       int i;
> +
> +retry:
> +       ep = READ_ONCE(early_pfn_freelist);
> +       if (ep) {
> +               struct early_pfn_node *next = READ_ONCE(ep->next);
> +
> +               if (try_cmpxchg(&early_pfn_freelist, &ep, next))
> +                       return ep;
> +               goto retry;
> +       }
> +
> +       page = alloc_page(gfp | __GFP_NO_CODETAG | __GFP_ZERO);
> +       if (!page)
> +               return NULL;
> +
> +       new = page_address(page);
> +       for (i = 0; i < NODES_PER_PAGE - 1; i++)
> +               new[i].next = &new[i + 1];
> +       new[NODES_PER_PAGE - 1].next = NULL;
> +
> +       if (cmpxchg(&early_pfn_freelist, NULL, new + 1)) {
> +               __free_page(page);
> +               goto retry;
> +       }
>
>         do {
> -               old_idx = atomic_read(&early_pfn_count);
> -               if (old_idx >= EARLY_ALLOC_PFN_MAX) {
> -                       pr_warn_once("Early page allocations before page_ext init exceeded EARLY_ALLOC_PFN_MAX (%d)\n",
> -                                     EARLY_ALLOC_PFN_MAX);
> -                       return;
> -               }
> -               new_idx = old_idx + 1;
> -       } while (!atomic_try_cmpxchg(&early_pfn_count, &old_idx, new_idx));
> +               old_page = READ_ONCE(early_pfn_pages);
> +               page->private = (unsigned long)old_page;
> +       } while (cmpxchg(&early_pfn_pages, old_page, page) != old_page);

I don't think this whole lockless schema is worth the complexity.
alloc_early_pfn_node() is called only during early init and is called
perhaps a few hundred times in total. Why not use a simple spinlock to
synchronize this operation and be done with it?

> +
> +       return new;
> +}
> +
> +static void __init __alloc_tag_add_early_pfn(unsigned long pfn, gfp_t gfp_flags)
> +{
> +       struct early_pfn_node *ep = alloc_early_pfn_node(gfp_flags);
>
> -       early_pfns[old_idx] = pfn;
> +       if (!ep)
> +               return;
> +
> +       ep->pfn = pfn;
> +       do {
> +               ep->next = READ_ONCE(early_pfn_list);
> +       } while (!try_cmpxchg(&early_pfn_list, &ep->next, ep));
>  }
>
> -typedef void alloc_tag_add_func(unsigned long pfn);
> +typedef void alloc_tag_add_func(unsigned long pfn, gfp_t gfp_flags);
>  static alloc_tag_add_func __rcu *alloc_tag_add_early_pfn_ptr __refdata =
>         RCU_INITIALIZER(__alloc_tag_add_early_pfn);
>
> -void alloc_tag_add_early_pfn(unsigned long pfn)
> +void alloc_tag_add_early_pfn(unsigned long pfn, gfp_t gfp_flags)
>  {
>         alloc_tag_add_func *alloc_tag_add;
>
> @@ -814,13 +844,14 @@ void alloc_tag_add_early_pfn(unsigned long pfn)
>         rcu_read_lock();
>         alloc_tag_add = rcu_dereference(alloc_tag_add_early_pfn_ptr);
>         if (alloc_tag_add)
> -               alloc_tag_add(pfn);
> +               alloc_tag_add(pfn, gfp_flags);
>         rcu_read_unlock();
>  }
>
>  static void __init clear_early_alloc_pfn_tag_refs(void)
>  {
> -       unsigned int i;
> +       struct early_pfn_node *ep;
> +       struct page *page, *next;
>
>         if (static_key_enabled(&mem_profiling_compressed))
>                 return;
> @@ -829,14 +860,13 @@ static void __init clear_early_alloc_pfn_tag_refs(void)
>         /* Make sure we are not racing with __alloc_tag_add_early_pfn() */
>         synchronize_rcu();
>
> -       for (i = 0; i < atomic_read(&early_pfn_count); i++) {
> -               unsigned long pfn = early_pfns[i];
> +       for (ep = early_pfn_list; ep; ep = ep->next) {
>
> -               if (pfn_valid(pfn)) {
> -                       struct page *page = pfn_to_page(pfn);
> +               if (pfn_valid(ep->pfn)) {
>                         union pgtag_ref_handle handle;
>                         union codetag_ref ref;
>
> +                       page = pfn_to_page(ep->pfn);
>                         if (get_page_tag_ref(page, &ref, &handle)) {
>                                 /*
>                                  * An early-allocated page could be freed and reallocated
> @@ -861,6 +891,12 @@ static void __init clear_early_alloc_pfn_tag_refs(void)
>                 }
>
>         }
> +
> +       for (page = early_pfn_pages; page; page = next) {
> +               next = (struct page *)page->private;
> +               clear_page_tag_ref(page);
> +               __free_page(page);
> +       }
>  }
>  #else /* !CONFIG_MEM_ALLOC_PROFILING_DEBUG */
>  static inline void __init clear_early_alloc_pfn_tag_refs(void) {}
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 04494bc2e46f..4e2bfb3714e1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1284,7 +1284,7 @@ void __clear_page_tag_ref(struct page *page)
>  /* 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 nr, gfp_t gfp_flags)
>  {
>         union pgtag_ref_handle handle;
>         union codetag_ref ref;
> @@ -1294,21 +1294,30 @@ void __pgalloc_tag_add(struct page *page, struct task_struct *task,
>                 update_page_tag_ref(handle, &ref);
>                 put_page_tag_ref(handle);
>         } else {
> -               /*
> -                * page_ext is not available yet, record the pfn so we can
> -                * clear the tag ref later when page_ext is initialized.
> -                */
> -               alloc_tag_add_early_pfn(page_to_pfn(page));
> +
>                 if (task->alloc_tag)
>                         alloc_tag_set_inaccurate(task->alloc_tag);
> +
> +               /*
> +                * page_ext is not available yet, skip if this allocation
> +                * doesn't need early PFN recording.
> +                */
> +               if (unlikely(!should_record_early_pfn(gfp_flags)))
> +                       return;
> +
> +               /*
> +                * Record the pfn so the tag ref can be cleared later
> +                * when page_ext is initialized.
> +                */
> +               alloc_tag_add_early_pfn(page_to_pfn(page), gfp_flags);

nit: This seems shorter and more readable:

               if (unlikely(should_record_early_pfn(gfp_flags)))
                            alloc_tag_add_early_pfn(page_to_pfn(page),
gfp_flags);

>         }
>  }
>
>  static inline void pgalloc_tag_add(struct page *page, struct task_struct *task,
> -                                  unsigned int nr)
> +                                  unsigned int nr, gfp_t gfp_flags)
>  {
>         if (mem_alloc_profiling_enabled())
> -               __pgalloc_tag_add(page, task, nr);
> +               __pgalloc_tag_add(page, task, nr, gfp_flags);
>  }
>
>  /* Should be called only if mem_alloc_profiling_enabled() */
> @@ -1341,7 +1350,7 @@ static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr)
>  #else /* CONFIG_MEM_ALLOC_PROFILING */
>
>  static inline void pgalloc_tag_add(struct page *page, struct task_struct *task,
> -                                  unsigned int nr) {}
> +                                  unsigned int nr, gfp_t gfp_flags) {}
>  static inline void pgalloc_tag_sub(struct page *page, unsigned int nr) {}
>  static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr) {}
>
> @@ -1896,7 +1905,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
>
>         set_page_owner(page, order, gfp_flags);
>         page_table_check_alloc(page, order);
> -       pgalloc_tag_add(page, current, 1 << order);
> +       pgalloc_tag_add(page, current, 1 << order, gfp_flags);
>  }
>
>  static void prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
> --
> 2.25.1
>