Re: [PATCH v5] mm/alloc_tag: replace fixed-size early PFN array with dynamic linked list

[Suren Baghdasaryan]

On Tue, May 26, 2026 at 10:22 PM Hao Ge <hao.ge@xxxxxxxxx> wrote:
>
>
> On 2026/5/27 10:00, Andrew Morton wrote:
> > On Fri, 8 May 2026 17:12:51 -0700 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> >> On Wed,  6 May 2026 10:22:56 +0800 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 pfn_pool structs. Each node is allocated via alloc_page() and mapped
> >>> to a pfn_pool containing a next pointer, an atomic slot counter, and a
> >>> PFN array that fills the remainder of the page.
> >>>
> >>> The tracking 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.
> >> AI review asked a couple of things.  I have a feeling we saw at least
> >> one of these, so probably already dealt with.
> >>      https://sashiko.dev/#/patchset/20260506022256.32664-1-hao.ge@xxxxxxxxx
>
> Hi Andrew
>
> My apologies. I'm also waiting for Suren's review. He may have been tied
> up lately
>
> and might not have time to get to this.

Sorry folks, I was on vacation and will be fully back to work
tomorrow. I'll start on these reviews the first thing once I'm at my
station.


>
>
> Sashiko raised two issues this time. I've already responded to the first
> one.
>
> See the link below:
>
> https://lore.kernel.org/all/0b9969e2-b208-46c2-a9a5-bf620239275a@xxxxxxxxx/
>
> If I haven't missed any details, it should be a false positive.
>
>
> As for the second point, let me address it.
>
> The early PFN tracking window is entirely within mm_core_init(),
>
> which is called from start_kernel():
>
> start_kernel()
>
>      mm_core_init()
>
>          memblock_free_all();
>
>          mem_init() //start early PFN tracking
>
>          kmem_cache_init()                           // SLUB bootstrap +
> kmalloc caches
> ...
>          page_ext_init()                                   // clears
> alloc_tag_add_early_pfn_ptr
>
>      ...
>
>      rest_init() //spawns kernel_init thread
>
>
> kernel_init() → kernel_init_freeable()            // separate thread, later
>
>      smp_init()                                    // secondary CPUs
> come online here
>
> Within the early PFN window (mem_init() to page_ext_init()):
>
>   1. We are still in start_kernel(), single CPU. The buddy allocator
>
> was just initialized from memblock and should have plenty of free
>
> pages, so alloc_page() would likely be satisfied from the fast
>
> path. If so, the __GFP_NOFAIL without __GFP_DIRECT_RECLAIM
>
> check in the slowpath would not be reached.
>
> 2. Since only the boot CPU is running, alloc_page() targets the
>
> boot node, which has memory. So even if __GFP_THISNODE were
>
> inherited, it would not fail on the boot node during this window.
>
>
> So Sashiko's analysis applies to the general case, and indeed the issues
>
> he raised could occur there.
>
> However, in the early boot scenario, I believe the current patch is safe,
>
> even though it is not fully generic (after all, no one can predict
> future use cases).
>
> Therefore, I agree with his suggestion that using a clean mask like
> GFP_NOWAIT | __GFP_NOWARN.
>
>
> In any case, I will wait for your and Suren's feedback. You may have
> different opinions on this matter.
>
>
> Thanks
>
> Best Regards
>
> Hao
>
>
> > Please?
> >
> > Also, this patch has no evidence of human review.
> >
> >
> > From: Hao Ge <hao.ge@xxxxxxxxx>
> > Subject: mm/alloc_tag: replace fixed-size early PFN array with dynamic linked list
> > Date: Wed, 6 May 2026 10:22:56 +0800
> >
> > 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
> > pfn_pool structs.  Each node is allocated via alloc_page() and mapped to a
> > pfn_pool containing a next pointer, an atomic slot counter, and a PFN
> > array that fills the remainder of the page.
> >
> > The tracking 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.
> >
> > Link: https://lore.kernel.org/20260506022256.32664-1-hao.ge@xxxxxxxxx
> > Signed-off-by: Hao Ge <hao.ge@xxxxxxxxx>
> > Suggested-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > Cc: Brendan Jackman <jackmanb@xxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
> > Cc: Zi Yan <ziy@xxxxxxxxxx>
> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > ---
> >
> >   include/linux/alloc_tag.h |    4
> >   lib/alloc_tag.c           |  145 +++++++++++++++++++++++-------------
> >   mm/page_alloc.c           |   12 +-
> >   3 files changed, 102 insertions(+), 59 deletions(-)
> >
> > --- a/include/linux/alloc_tag.h~mm-alloc_tag-replace-fixed-size-early-pfn-array-with-dynamic-linked-list
> > +++ a/include/linux/alloc_tag.h
> > @@ -163,11 +163,11 @@ static inline void alloc_tag_sub_check(u
> >   {
> >       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) {}
> >   #endif
> >
> >   /* Caller should verify both ref and tag to be valid */
> > --- a/lib/alloc_tag.c~mm-alloc_tag-replace-fixed-size-early-pfn-array-with-dynamic-linked-list
> > +++ a/lib/alloc_tag.c
> > @@ -767,60 +767,95 @@ static __init bool need_page_alloc_taggi
> >    * 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.
> > + * Each page is cast to a pfn_pool: the first few bytes hold metadata
> > + * (next pointer and slot count), the remainder stores PFNs.
> > + */
> > +struct pfn_pool {
> > +     struct pfn_pool *next;
> > +     atomic_t count;
> > +     unsigned long pfns[];
> > +};
> > +
> > +#define PFN_POOL_SIZE                        ((PAGE_SIZE - offsetof(struct pfn_pool, pfns)) / \
> > +                                      sizeof(unsigned long))
> > +
> > +/*
> > + * 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.
> >    *
> > - * TODO: Replace fixed-size array with dynamic allocation using
> > - * a GFP flag similar to ___GFP_NO_OBJ_EXT to avoid recursion.
> > + * 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 EARLY_ALLOC_PFN_MAX          8192
> > +#define __GFP_NO_CODETAG             __GFP_NO_OBJ_EXT
> >
> > -static unsigned long early_pfns[EARLY_ALLOC_PFN_MAX] __initdata;
> > -static atomic_t early_pfn_count __initdata = ATOMIC_INIT(0);
> > +static struct pfn_pool *current_pfn_pool __initdata;
> >
> > -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 pfn_pool *pool;
> > +     int 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;
> > +             pool = READ_ONCE(current_pfn_pool);
> > +             if (!pool || atomic_read(&pool->count) >= PFN_POOL_SIZE) {
> > +                     gfp_t gfp = gfp_flags & ~(__GFP_DIRECT_RECLAIM | GFP_ZONEMASK);
> > +                     struct page *new_page = alloc_page(gfp | __GFP_NO_CODETAG);
> > +                     struct pfn_pool *new;
> > +
> > +                     if (!new_page) {
> > +                             pr_warn_once("early PFN tracking page allocation failed\n");
> > +                             return;
> > +                     }
> > +                     new = page_address(new_page);
> > +                     new->next = pool;
> > +                     atomic_set(&new->count, 0);
> > +                     if (cmpxchg(&current_pfn_pool, pool, new) != pool) {
> > +                             clear_page_tag_ref(new_page);
> > +                             __free_page(new_page);
> > +                             continue;
> > +                     }
> > +                     pool = new;
> >               }
> > -             new_idx = old_idx + 1;
> > -     } while (!atomic_try_cmpxchg(&early_pfn_count, &old_idx, new_idx));
> > +             idx = atomic_read(&pool->count);
> > +             if (idx >= PFN_POOL_SIZE)
> > +                     continue;
> > +             if (atomic_cmpxchg(&pool->count, idx, idx + 1) == idx)
> > +                     break;
> > +     } while (1);
> >
> > -     early_pfns[old_idx] = pfn;
> > +     pool->pfns[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;
> >
> >       if (static_key_enabled(&mem_profiling_compressed))
> >               return;
> >
> > +     /* Skip allocations for the tracking list itself to avoid recursion. */
> > +     if (gfp_flags & __GFP_NO_CODETAG)
> > +             return;
> > +
> >       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 pfn_pool *pool, *next;
> > +     struct page *page;
> > +     int i;
> >
> >       if (static_key_enabled(&mem_profiling_compressed))
> >               return;
> > @@ -829,37 +864,45 @@ static void __init clear_early_alloc_pfn
> >       /* 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 (pool = current_pfn_pool; pool; pool = next) {
> > +             int nr_pfns = atomic_read(&pool->count);
> > +
> > +             for (i = 0; i < nr_pfns; i++) {
> > +                     unsigned long pfn = pool->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) {
> > +                     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 = pool->next;
> > +             page = virt_to_page(pool);
> > +             clear_page_tag_ref(page);
> > +             __free_page(page);
> >       }
> >   }
> >   #else /* !CONFIG_MEM_ALLOC_PROFILING_DEBUG */
> > --- a/mm/page_alloc.c~mm-alloc_tag-replace-fixed-size-early-pfn-array-with-dynamic-linked-list
> > +++ a/mm/page_alloc.c
> > @@ -1255,7 +1255,7 @@ void __clear_page_tag_ref(struct page *p
> >   /* 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;
> > @@ -1269,17 +1269,17 @@ void __pgalloc_tag_add(struct page *page
> >                * 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));
> > +             alloc_tag_add_early_pfn(page_to_pfn(page), gfp_flags);
> >               if (task->alloc_tag)
> >                       alloc_tag_set_inaccurate(task->alloc_tag);
> >       }
> >   }
> >
> >   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() */
> > @@ -1312,7 +1312,7 @@ static inline void pgalloc_tag_sub_pages
> >   #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) {}
> >
> > @@ -1867,7 +1867,7 @@ inline void post_alloc_hook(struct page
> >
> >       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,
> > _
> >