From: Hao Ge <hao.ge@linux.dev>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] mm/alloc_tag: replace fixed-size early PFN array with dynamic linked list
Date: Tue, 28 Apr 2026 11:00:10 +0800 [thread overview]
Message-ID: <b4735edb-55f9-4562-a0d0-a10e1788979a@linux.dev> (raw)
In-Reply-To: <CAJuCfpEy=rDFRhS+X-a5t-UquPi7veMRmyZzxR_9Z-WKagwFaw@mail.gmail.com>
Hi Suren
Thanks a lot for your review and suggestions.
On 2026/4/28 00:02, Suren Baghdasaryan wrote:
> On Thu, Apr 23, 2026 at 1:39 AM 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
>> 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@linux.dev>
>> ---
>> 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.
That's reasonable.
I will adjust the wording in v2.
>> + */
>> +#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.
Adding a struct llist_node into the page union only for the
CONFIG_MEM_ALLOC_PROFILING_DEBUG feature,
which may not be quite appropriate from a design perspective.
> 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;
>> + }
I also considered using llist_head initially. like this
static LLIST_HEAD(early_pfn_pages);
static void __init __alloc_tag_add_early_pfn(unsigned long pfn, gfp_t
gfp_flags)
{
struct page *page;
unsigned long idx;
do {
page = llist_first(&early_pfn_pages);
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->private = 0;
llist_add(&new->pcp_llist, &early_pfn_pages);
page = new;
}
idx = READ_ONCE(page->private);
if (idx >= EARLY_PFNS_PER_PAGE)
continue;
if (cmpxchg(&page->private, idx, idx + 1) == idx)
break;
} while (1);
((unsigned long *)page_address(page))[idx] = pfn;
}
The problem is that llist_add() retries internally until it succeeds.
When multiple CPUs concurrently see that the head page is NULL or full ,
they each allocate a new tracking page and all insertions succeed:
CPU0: head page A is full or NULL, allocates B, llist_add(B) → head=B→A
CPU1: head page A is full or NULL, allocates C, llist_add(C) → head=C→B→A
CPU0: claims slot 0 in B
CPU1: claims slot 0 in C
Now the head is C. Once C fills up, a new page D is allocated and
becomes the new head. But B still has ~511 unused slots and will never
be the head again — we only ever look at llist_first(). Those slots are
permanently wasted.
That's why I used struct page *early_pfn_current with direct cmpxchg —
the loser frees its page and retries, avoiding the waste.
That said, I may be missing something. Is there a way to use llist that
avoids this? I'd appreciate your thoughts.
>> + 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;
will fix in the next version.
>
>> + } 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.
will fix in the next version.
>> +
>> + 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.
>
will fix in the next version.
Thanks
Best regards.
Hao
>> }
>> }
>>
>> 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
>>
prev parent reply other threads:[~2026-04-28 3:01 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-04-23 8:37 [PATCH v3] mm/alloc_tag: replace fixed-size early PFN array with dynamic linked list Hao Ge
2026-04-27 16:02 ` Suren Baghdasaryan
2026-04-27 18:23 ` Suren Baghdasaryan
2026-04-28 3:00 ` Hao Ge [this message]
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