Re: [PATCH v3] mm/alloc_tag: replace fixed-size early PFN array with dynamic linked list

[Suren Baghdasaryan]

On Thu, Apr 23, 2026 at 1:39 AM Hao Ge <hao.ge@xxxxxxxxx> 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
> of pages. Each page stores PFNs in its page body and uses page->lru
> to link to the next page, with page->private tracking the number of
> used slots.
>
> The list pages themselves are allocated via alloc_page(), which would
> trigger __pgalloc_tag_add() -> alloc_tag_add_early_pfn() 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.
>
> Signed-off-by: Hao Ge <hao.ge@xxxxxxxxx>
> ---
> v3:
> - Simplify linked list: use page->lru for chaining and page->private as
>   slot counter, removing the early_pfn_node struct and freelist (suggested
>   by Suren Baghdasaryan)
> - Pass gfp_flags through alloc_tag_add_early_pfn() but strip
>   __GFP_DIRECT_RECLAIM instead of selecting GFP_KERNEL/GFP_ATOMIC,
>   because __alloc_tag_add_early_pfn() is invoked under rcu_read_lock().
>
> 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           | 118 +++++++++++++++++++++-----------------
>  mm/page_alloc.c           |  23 ++++----
>  3 files changed, 97 insertions(+), 66 deletions(-)
>
> diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
> index 02de2ede560f..ce9b478033bf 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_tag_add_early_pfn() 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.

nit: Callers must... sounds like someone who uses
__alloc_tag_add_early_pfn() has to do something extra, however
clear_page_tag_ref() is done by clear_early_alloc_pfn_tag_refs(), so
callers don't really need to do this. I suggest rephrasing as:

Codetags of the pages allocated with __GFP_NO_CODETAG should be
cleared (via clear_page_tag_ref()) before freeing the pages to prevent
alloc_tag_sub_check() from triggering 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 false; }
>  #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..a00dc5a867e4 100644
> --- a/lib/alloc_tag.c
> +++ b/lib/alloc_tag.c
> @@ -766,45 +766,49 @@ 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
> +#define EARLY_PFNS_PER_PAGE            (PAGE_SIZE / sizeof(unsigned long))
>
> -static unsigned long early_pfns[EARLY_ALLOC_PFN_MAX] __initdata;
> -static atomic_t early_pfn_count __initdata = ATOMIC_INIT(0);
> +static struct page *early_pfn_current __initdata;

So, instead of using early_pfn_current as the head page and then
page->lru.next directly, I would suggest having "struct llist_head
early_pfn_head;" as the list head and either using page->pcp_llist to
link the pages or even better, adding a "struct llist_node llist;"
into the union containing "struct list_head lru;". Note that
llist_add() uses try_cmpxchg(), so the link addition is atomic.
And you should probably use folios rather than pages to follow the new trends.

>
> -static void __init __alloc_tag_add_early_pfn(unsigned long pfn)
> +static void __init __alloc_tag_add_early_pfn(unsigned long pfn, gfp_t gfp_flags)
>  {
> -       int old_idx, new_idx;
> +       struct page *page;
> +       unsigned long idx;
>
>         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;
> +               page = READ_ONCE(early_pfn_current);
> +               if (!page || READ_ONCE(page->private) >= EARLY_PFNS_PER_PAGE) {
> +                       gfp_t gfp = gfp_flags & ~__GFP_DIRECT_RECLAIM;
> +                       struct page *new = alloc_page(gfp | __GFP_NO_CODETAG);
> +
> +                       if (!new) {
> +                               pr_warn_once("early PFN tracking page allocation failed\n");
> +                               return;
> +                       }
> +                       new->lru.next = (struct list_head *)page;
> +                       if (cmpxchg(&early_pfn_current, page, new) != page) {
> +                               __free_page(new);
> +                               continue;
> +                       }
> +                       page = new;
>                 }
> -               new_idx = old_idx + 1;
> -       } while (!atomic_try_cmpxchg(&early_pfn_count, &old_idx, new_idx));
> +               idx = READ_ONCE(page->private);
> +               if (idx >= EARLY_PFNS_PER_PAGE)
> +                       continue;
> +               if (cmpxchg(&page->private, idx, idx + 1) != idx)
> +                       continue;
> +               break;

nit: This would read more easily:

               if (cmpxchg(&page->private, idx, idx + 1) == idx)
                        break;


> +       } while (1);
>
> -       early_pfns[old_idx] = pfn;
> +       ((unsigned long *)page_address(page))[idx] = pfn;
>  }
>
> -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 +818,14 @@ 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 page *page, *next;
> +       unsigned long i;
>
>         if (static_key_enabled(&mem_profiling_compressed))
>                 return;
> @@ -829,37 +834,42 @@ 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];
> -
> -               if (pfn_valid(pfn)) {
> -                       struct page *page = pfn_to_page(pfn);
> -                       union pgtag_ref_handle handle;
> -                       union codetag_ref ref;
> -
> -                       if (get_page_tag_ref(page, &ref, &handle)) {
> -                               /*
> -                                * An early-allocated page could be freed and reallocated
> -                                * after its page_ext is initialized but before we clear it.
> -                                * In that case, it already has a valid tag set.
> -                                * We should not overwrite that valid tag with CODETAG_EMPTY.
> -                                *
> -                                * Note: there is still a small race window between checking
> -                                * ref.ct and calling set_codetag_empty(). We accept this
> -                                * race as it's unlikely and the extra complexity of atomic
> -                                * cmpxchg is not worth it for this debug-only code path.
> -                                */
> -                               if (ref.ct) {
> +       for (page = early_pfn_current; page; page = next) {
> +               for (i = 0; i < page->private; i++) {
> +                       unsigned long pfn = ((unsigned long *)page_address(page))[i];

This page_address() can be done in the outer loop since the page does
not change inside the inner loop.

> +
> +                       if (pfn_valid(pfn)) {
> +                               union pgtag_ref_handle handle;
> +                               union codetag_ref ref;
> +
> +                               if (get_page_tag_ref(pfn_to_page(pfn), &ref, &handle)) {
> +                                       /*
> +                                        * An early-allocated page could be freed and reallocated
> +                                        * after its page_ext is initialized but before we clear it.
> +                                        * In that case, it already has a valid tag set.
> +                                        * We should not overwrite that valid tag
> +                                        * with CODETAG_EMPTY.
> +                                        *
> +                                        * Note: there is still a small race window between checking
> +                                        * ref.ct and calling set_codetag_empty(). We accept this
> +                                        * race as it's unlikely and the extra complexity of atomic
> +                                        * cmpxchg is not worth it for this debug-only code path.
> +                                        */
> +                                       if (ref.ct) {
> +                                               put_page_tag_ref(handle);
> +                                               continue;
> +                                       }
> +
> +                                       set_codetag_empty(&ref);
> +                                       update_page_tag_ref(handle, &ref);
>                                         put_page_tag_ref(handle);
> -                                       continue;
>                                 }
> -
> -                               set_codetag_empty(&ref);
> -                               update_page_tag_ref(handle, &ref);
> -                               put_page_tag_ref(handle);
>                         }
>                 }
>
> +               next = (struct page *)page->lru.next;
> +               clear_page_tag_ref(page);
> +               __free_page(page);
>         }
>  }
>  #else /* !CONFIG_MEM_ALLOC_PROFILING_DEBUG */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 04494bc2e46f..5b7b234967a5 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 +1294,24 @@ 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, record the pfn so the
> +                * tag ref can be cleared later when page_ext is initialized.
> +                */
> +               if (should_record_early_pfn(gfp_flags))
> +                       alloc_tag_add_early_pfn(page_to_pfn(page), gfp_flags);

Any reason for changing the order of alloc_tag_add_early_pfn() and
alloc_tag_set_inaccurate()? I liked the previous version where we
explain why this is done at the very beginning of the block. You could
also call should_record_early_pfn() from inside
alloc_tag_add_early_pfn(), which would keep both
should_record_early_pfn() and __GFP_NO_CODETAG definitions local to
alloc_tag.c.

>         }
>  }
>
>  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 +1344,7 @@ 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 +1899,7 @@ 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
>