Re: [PATCH] mm/alloc_tag: clear codetag for pages allocated before page_ext initialization

Suren Baghdasaryan
2026-03-20T02:14:53Z

On Thu, Mar 19, 2026 at 6:58 PM Hao Ge <hao.ge@linux.dev> wrote:
>
>
> On 2026/3/20 07:48, Suren Baghdasaryan wrote:
> > On Thu, Mar 19, 2026 at 4:44 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >> On Thu, Mar 19, 2026 at 3:28 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >>> On Thu, 19 Mar 2026 16:31:53 +0800 Hao Ge <hao.ge@linux.dev> wrote:
> >>>
> >>>> Due to initialization ordering, page_ext is allocated and initialized
> >>>> relatively late during boot. Some pages have already been allocated
> >>>> and freed before page_ext becomes available, leaving their codetag
> >>>> uninitialized.
> >> Hi Hao,
> >> Thanks for the report.
> >> Hmm. So, we are allocating pages before page_ext is initialized...
> >>
> >>>> A clear example is in init_section_page_ext(): alloc_page_ext() calls
> >>>> kmemleak_alloc().
> > Forgot to ask. The example you are using here is for page_ext
> > allocation itself. Do you have any other examples where page
> > allocation happens before page_ext initialization? If that's the only
> > place, then we might be able to fix this in a simpler way by doing
> > something special for alloc_page_ext().
>
> Hi Suren
>
> To help illustrate the point, here's the debug log I added:
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2d4b6f1a554e..ebfe636f5b07 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1293,6 +1293,9 @@ void __pgalloc_tag_add(struct page *page, struct
> task_struct *task,
> alloc_tag_add(&ref, task->alloc_tag, PAGE_SIZE * nr);
> update_page_tag_ref(handle, &ref);
> put_page_tag_ref(handle);
> + } else {
> + pr_warn("__pgalloc_tag_add: get_page_tag_ref failed!
> page=%p pfn=%lu nr=%u\n", page, page_to_pfn(page), nr);
> + dump_stack();
> }
> }
>
>
> And I caught the following logs:
>
> [ 0.296399] __pgalloc_tag_add: get_page_tag_ref failed!
> page=ffffea000400c700 pfn=1049372 nr=1
> [ 0.296400] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted
> 7.0.0-rc4-dirty #12 PREEMPT(lazy)
> [ 0.296402] Hardware name: Red Hat KVM, BIOS
> rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
> [ 0.296402] Call Trace:
> [ 0.296403] <TASK>
> [ 0.296403] dump_stack_lvl+0x53/0x70
> [ 0.296405] __pgalloc_tag_add+0x3a3/0x6e0
> [ 0.296406] ? __pfx___pgalloc_tag_add+0x10/0x10
> [ 0.296407] ? kasan_unpoison+0x27/0x60
> [ 0.296409] ? __kasan_unpoison_pages+0x2c/0x40
> [ 0.296411] get_page_from_freelist+0xa54/0x1310
> [ 0.296413] __alloc_frozen_pages_noprof+0x206/0x4c0
> [ 0.296415] ? __pfx___alloc_frozen_pages_noprof+0x10/0x10
> [ 0.296417] ? stack_depot_save_flags+0x3f/0x680
> [ 0.296418] ? ___slab_alloc+0x518/0x530
> [ 0.296420] alloc_pages_mpol+0x13a/0x3f0
> [ 0.296421] ? __pfx_alloc_pages_mpol+0x10/0x10
> [ 0.296423] ? _raw_spin_lock_irqsave+0x8a/0xf0
> [ 0.296424] ? __pfx__raw_spin_lock_irqsave+0x10/0x10
> [ 0.296426] alloc_slab_page+0xc2/0x130
> [ 0.296427] allocate_slab+0x77/0x2c0
> [ 0.296429] ? syscall_enter_define_fields+0x3bb/0x5f0
> [ 0.296430] ___slab_alloc+0x125/0x530
> [ 0.296432] ? __trace_define_field+0x252/0x3d0
> [ 0.296433] __kmalloc_noprof+0x329/0x630
> [ 0.296435] ? syscall_enter_define_fields+0x3bb/0x5f0
> [ 0.296436] syscall_enter_define_fields+0x3bb/0x5f0
> [ 0.296438] ? __pfx_syscall_enter_define_fields+0x10/0x10
> [ 0.296440] event_define_fields+0x326/0x540
> [ 0.296441] __trace_early_add_events+0xac/0x3c0
> [ 0.296443] trace_event_init+0x24c/0x460
> [ 0.296445] trace_init+0x9/0x20
> [ 0.296446] start_kernel+0x199/0x3c0
> [ 0.296448] x86_64_start_reservations+0x18/0x30
> [ 0.296449] x86_64_start_kernel+0xe2/0xf0
> [ 0.296451] common_startup_64+0x13e/0x141
> [ 0.296453] </TASK>
>
>
> [ 0.312234] __pgalloc_tag_add: get_page_tag_ref failed!
> page=ffffea000400f900 pfn=1049572 nr=1
> [ 0.312234] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted
> 7.0.0-rc4-dirty #12 PREEMPT(lazy)
> [ 0.312236] Hardware name: Red Hat KVM, BIOS
> rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
> [ 0.312236] Call Trace:
> [ 0.312237] <TASK>
> [ 0.312237] dump_stack_lvl+0x53/0x70
> [ 0.312239] __pgalloc_tag_add+0x3a3/0x6e0
> [ 0.312240] ? __pfx___pgalloc_tag_add+0x10/0x10
> [ 0.312241] ? rmqueue.constprop.0+0x4fc/0x1ce0
> [ 0.312243] ? kasan_unpoison+0x27/0x60
> [ 0.312244] ? __kasan_unpoison_pages+0x2c/0x40
> [ 0.312246] get_page_from_freelist+0xa54/0x1310
> [ 0.312248] __alloc_frozen_pages_noprof+0x206/0x4c0
> [ 0.312250] ? __pfx___alloc_frozen_pages_noprof+0x10/0x10
> [ 0.312253] alloc_slab_page+0x39/0x130
> [ 0.312254] allocate_slab+0x77/0x2c0
> [ 0.312255] ? alloc_cpumask_var_node+0xc7/0x230
> [ 0.312257] ___slab_alloc+0x46d/0x530
> [ 0.312259] __kmalloc_node_noprof+0x2fa/0x680
> [ 0.312261] ? alloc_cpumask_var_node+0xc7/0x230
> [ 0.312263] alloc_cpumask_var_node+0xc7/0x230
> [ 0.312264] init_desc+0x141/0x6b0
> [ 0.312266] alloc_desc+0x108/0x1b0
> [ 0.312267] early_irq_init+0xee/0x1c0
> [ 0.312268] ? __pfx_early_irq_init+0x10/0x10
> [ 0.312271] start_kernel+0x1ab/0x3c0
> [ 0.312272] x86_64_start_reservations+0x18/0x30
> [ 0.312274] x86_64_start_kernel+0xe2/0xf0
> [ 0.312275] common_startup_64+0x13e/0x141
> [ 0.312277] </TASK>
>
> [ 0.312834] __pgalloc_tag_add: get_page_tag_ref failed!
> page=ffffea000400fc00 pfn=1049584 nr=1
> [ 0.312835] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted
> 7.0.0-rc4-dirty #12 PREEMPT(lazy)
> [ 0.312836] Hardware name: Red Hat KVM, BIOS
> rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
> [ 0.312837] Call Trace:
> [ 0.312837] <TASK>
> [ 0.312838] dump_stack_lvl+0x53/0x70
> [ 0.312840] __pgalloc_tag_add+0x3a3/0x6e0
> [ 0.312841] ? __pfx___pgalloc_tag_add+0x10/0x10
> [ 0.312842] ? rmqueue.constprop.0+0x4fc/0x1ce0
> [ 0.312844] ? kasan_unpoison+0x27/0x60
> [ 0.312845] ? __kasan_unpoison_pages+0x2c/0x40
> [ 0.312847] get_page_from_freelist+0xa54/0x1310
> [ 0.312849] __alloc_frozen_pages_noprof+0x206/0x4c0
> [ 0.312851] ? __pfx___alloc_frozen_pages_noprof+0x10/0x10
> [ 0.312853] alloc_pages_mpol+0x13a/0x3f0
> [ 0.312855] ? __pfx_alloc_pages_mpol+0x10/0x10
> [ 0.312856] ? xas_find+0x2d8/0x450
> [ 0.312858] ? _raw_spin_lock+0x84/0xe0
> [ 0.312859] ? __pfx__raw_spin_lock+0x10/0x10
> [ 0.312861] alloc_pages_noprof+0xf6/0x2b0
> [ 0.312862] __change_page_attr+0x293/0x850
> [ 0.312864] ? __pfx___change_page_attr+0x10/0x10
> [ 0.312865] ? _vm_unmap_aliases+0x2d0/0x650
> [ 0.312868] ? __pfx__vm_unmap_aliases+0x10/0x10
> [ 0.312869] __change_page_attr_set_clr+0x16c/0x360
> [ 0.312871] ? spp_getpage+0xbb/0x1e0
> [ 0.312872] change_page_attr_set_clr+0x220/0x3c0
> [ 0.312873] ? flush_tlb_one_kernel+0xf/0x30
> [ 0.312875] ? set_pte_vaddr_p4d+0x110/0x180
> [ 0.312877] ? __pfx_change_page_attr_set_clr+0x10/0x10
> [ 0.312878] ? __pfx_set_pte_vaddr_p4d+0x10/0x10
> [ 0.312881] ? __pfx_mtree_load+0x10/0x10
> [ 0.312883] ? __pfx_mtree_load+0x10/0x10
> [ 0.312884] ? __asan_memcpy+0x3c/0x60
> [ 0.312886] ? set_intr_gate+0x10c/0x150
> [ 0.312888] set_memory_ro+0x76/0xa0
> [ 0.312889] ? __pfx_set_memory_ro+0x10/0x10
> [ 0.312891] idt_setup_apic_and_irq_gates+0x2c1/0x390
>
> and more.

Ok, it's not the only place. Got your point.

>
> off topic - if we were to handle only alloc_page_ext() specifically,
> what would be the most straightforward
>
> solution in your mind? I'd really appreciate your insight.

I was thinking if it's the only special case maybe we can handle it
somehow differently, like we do when we allocate obj_ext vectors for
slabs using __GFP_NO_OBJ_EXT. I haven't found a good solution yet but
since it's not a special case we would not be able to use it even if I
came up with something...
I think your way is the most straight-forward but please try my
suggestion to see if we can avoid extra overhead.
Thanks,
Suren.

>
> Thanks.
>
>
> >>>> If the slab cache has no free objects, it falls back
> >>>> to the buddy allocator to allocate memory. However, at this point page_ext
> >>>> is not yet fully initialized, so these newly allocated pages have no
> >>>> codetag set. These pages may later be reclaimed by KASAN,which causes
> >>>> the warning to trigger when they are freed because their codetag ref is
> >>>> still empty.
> >>>>
> >>>> Use a global array to track pages allocated before page_ext is fully
> >>>> initialized, similar to how kmemleak tracks early allocations.
> >>>> When page_ext initialization completes, set their codetag
> >>>> to empty to avoid warnings when they are freed later.
> >>>>
> >>>> ...
> >>>>
> >>>> --- a/include/linux/alloc_tag.h
> >>>> +++ b/include/linux/alloc_tag.h
> >>>> @@ -74,6 +74,9 @@ static inline void set_codetag_empty(union codetag_ref *ref)
> >>>>
> >>>> #ifdef CONFIG_MEM_ALLOC_PROFILING
> >>>>
> >>>> +bool mem_profiling_is_available(void);
> >>>> +void alloc_tag_add_early_pfn(unsigned long pfn);
> >>>> +
> >>>> #define ALLOC_TAG_SECTION_NAME "alloc_tags"
> >>>>
> >>>> struct codetag_bytes {
> >>>> diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> >>>> index 58991ab09d84..a5bf4e72c154 100644
> >>>> --- a/lib/alloc_tag.c
> >>>> +++ b/lib/alloc_tag.c
> >>>> @@ -6,6 +6,7 @@
> >>>> #include <linux/kallsyms.h>
> >>>> #include <linux/module.h>
> >>>> #include <linux/page_ext.h>
> >>>> +#include <linux/pgalloc_tag.h>
> >>>> #include <linux/proc_fs.h>
> >>>> #include <linux/seq_buf.h>
> >>>> #include <linux/seq_file.h>
> >>>> @@ -26,6 +27,82 @@ static bool mem_profiling_support;
> >>>>
> >>>> static struct codetag_type *alloc_tag_cttype;
> >>>>
> >>>> +/*
> >>>> + * State of the alloc_tag
> >>>> + *
> >>>> + * This is used to describe the states of the alloc_tag during bootup.
> >>>> + *
> >>>> + * When we need to allocate page_ext to store codetag, we face an
> >>>> + * initialization timing problem:
> >>>> + *
> >>>> + * Due to initialization order, pages may be allocated via buddy system
> >>>> + * before page_ext is fully allocated and initialized. Although these
> >>>> + * pages call the allocation hooks, the codetag will not be set because
> >>>> + * page_ext is not yet available.
> >>>> + *
> >>>> + * When these pages are later free to the buddy system, it triggers
> >>>> + * warnings because their codetag is actually empty if
> >>>> + * CONFIG_MEM_ALLOC_PROFILING_DEBUG is enabled.
> >>>> + *
> >>>> + * Additionally, in this situation, we cannot record detailed allocation
> >>>> + * information for these pages.
> >>>> + */
> >>>> +enum mem_profiling_state {
> >>>> + DOWN, /* No mem_profiling functionality yet */
> >>>> + UP /* Everything is working */
> >>>> +};
> >>>> +
> >>>> +static enum mem_profiling_state mem_profiling_state = DOWN;
> >>>> +
> >>>> +bool mem_profiling_is_available(void)
> >>>> +{
> >>>> + return mem_profiling_state == UP;
> >>>> +}
> >>>> +
> >>>> +#ifdef CONFIG_MEM_ALLOC_PROFILING_DEBUG
> >>>> +
> >>>> +#define EARLY_ALLOC_PFN_MAX 256
> >>>> +
> >>>> +static unsigned long early_pfns[EARLY_ALLOC_PFN_MAX];
> >>> It's unfortunate that this isn't __initdata.
> >>>
> >>>> +static unsigned int early_pfn_count;
> >>>> +static DEFINE_SPINLOCK(early_pfn_lock);
> >>>> +
> >>>>
> >>>> ...
> >>>>
> >>>> --- a/mm/page_alloc.c
> >>>> +++ b/mm/page_alloc.c
> >>>> @@ -1293,6 +1293,13 @@ void __pgalloc_tag_add(struct page *page, struct task_struct *task,
> >>>> alloc_tag_add(&ref, task->alloc_tag, PAGE_SIZE * nr);
> >>>> update_page_tag_ref(handle, &ref);
> >>>> put_page_tag_ref(handle);
> >>>> + } else {
> >> This branch can be marked as "unlikely".
> >>
> >>>> + /*
> >>>> + * page_ext is not available yet, record the pfn so we can
> >>>> + * clear the tag ref later when page_ext is initialized.
> >>>> + */
> >>>> + if (!mem_profiling_is_available())
> >>>> + alloc_tag_add_early_pfn(page_to_pfn(page));
> >>>> }
> >>>> }
> >>> All because of this, I believe. Is this fixable?
> >>>
> >>> If we take that `else', we know we're running in __init code, yes? I
> >>> don't see how `__init pgalloc_tag_add_early()' could be made to work.
> >>> hrm. Something clever, please.
> >> We can have a pointer to a function that is initialized to point to
> >> alloc_tag_add_early_pfn, which is defined as __init and uses
> >> early_pfns which now can be defined as __initdata. After
> >> clear_early_alloc_pfn_tag_refs() is done we reset that pointer to
> >> NULL. __pgalloc_tag_add() instead of calling alloc_tag_add_early_pfn()
> >> directly checks that pointer and if it's not NULL then calls the
> >> function that it points to. This way __pgalloc_tag_add() which is not
> >> an __init function will be invoking alloc_tag_add_early_pfn() __init
> >> function only until we are done with initialization. I haven't tried
> >> this but I think that should work. This also eliminates the need for
> >> mem_profiling_state variable since we can use this function pointer
> >> instead.
> >>
> >>