Re: [PATCH v3] mm: don't schedule deferred kernel page table freeing while booting

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Fri, Sep 25, 2026 at 10:06:47AM +0500, Mikhail Gavrilov wrote:
> Booting with a boot-time function tracer and a filter, for example
>
>   ftrace=function ftrace_filter=pud_free_pmd_page
>
> panics on 7.3-rc4 as soon as the tracer starts:
>
>   [   23.531178] Starting tracer 'function'
>   [   23.675800] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000038: 0000 [#1] SMP KASAN NOPTI
>   [   23.819917] KASAN: null-ptr-deref in range [0x00000000000001c0-0x00000000000001c7]
>   [   23.964025] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 7.3.0-rc4-fe2ec83746e5-with-fixes-v2+ #195 PREEMPT(undef)
>   [   24.252248] RIP: 0010:__queue_work+0xab/0xf00
>   [   25.981629] Call Trace:
>   [   26.125727]  <TASK>
>   [   26.413912]  ? pagetable_free_kernel+0x20/0x120
>   [   26.990283]  queue_work_on+0x97/0xf0
>   [   27.134382]  __cpa_collapse_large_pages+0x501/0x6f0
>   [   27.566662]  cpa_flush+0x394/0x620
>   [   27.998953]  change_page_attr_set_clr+0x321/0x4a0
>   [   29.151729]  set_memory_rox+0xa2/0xf0
>   [   29.584018]  create_trampoline+0x431/0x6f0
>   ...
>   [   44.343347] Kernel panic - not syncing: Attempted to kill the idle task!
>
> The boot-time tracer is started from early_trace_init(), which runs
> before workqueue_init_early().  Making its trampoline read-only splits a
> large page, and CPA collapses it again right away.  The split table has
> been a kernel page table since commit 9e4a3ec3411b
> ("x86/mm/pat: Allocate split page tables as kernel page tables"), so the
> collapse frees it through pagetable_free_kernel(), which queues work on
> system_percpu_wq - still NULL at that point.  That commit is correct in
> itself; it only lets CPA reach pagetable_free_kernel() before the
> workqueue that function relies on exists.
>
> Keep putting the table on the list, but don't schedule the work while
> the system is still booting.  A core_initcall schedules it once to free
> whatever was queued by then.
>
> Fixes: 9e4a3ec3411b ("x86/mm/pat: Allocate split page tables as kernel page tables")
> Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
> Suggested-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> Cc: stable@vger.kernel.org
> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>

LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> Link: https://lore.kernel.org/20260924064321.23787-1-mikhail.v.gavrilov@gmail.com
> ---
> v3:
>  - Schedule the work once from a core_initcall to free whatever was
>    queued during boot (Lorenzo Stoakes, Dave Hansen, David Hildenbrand).
>    late_initcall would work just as well; workqueues exist from
>    workqueue_init() on.
>  - Keep the fix in pagetable_free_kernel() rather than skipping the
>    collapse during boot (Mike Rapoport): that would only avoid this
>    caller, and any other early free would still need a workqueue.
> v2: https://lore.kernel.org/20260924092307.22813-1-mikhail.v.gavrilov@gmail.com
> v1: https://lore.kernel.org/20260924064321.23787-1-mikhail.v.gavrilov@gmail.com
>
> Tested on a Ryzen 9 7950X with a Radeon RX 7900 XTX (lockdep, KASAN),
> 7.3-rc4 plus unrelated local changes, by booting with
>
>   ftrace=function ftrace_filter=pud_free_pmd_page,pagetable_free_kernel,kernel_pgtable_work_func,kernel_pgtable_drain_early
>
> The boot that panicked without the fix completes, and the trace shows
> kernel_pgtable_drain_early() and then kernel_pgtable_work_func() before
> any other kernel page table is freed.
>
>  mm/pgtable-generic.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index b91b1a98029c..cd227fc05d2d 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -438,12 +438,30 @@ static void kernel_pgtable_work_func(struct work_struct *work)
>  		__pagetable_free(pt);
>  }
>
> +static void schedule_kernel_pgtable_free(void)
> +{
> +	schedule_work(&kernel_pgtable_work.work);
> +}
> +
>  void pagetable_free_kernel(struct ptdesc *pt)
>  {
>  	spin_lock(&kernel_pgtable_work.lock);
>  	list_add(&pt->pt_list, &kernel_pgtable_work.list);
>  	spin_unlock(&kernel_pgtable_work.lock);
>
> -	schedule_work(&kernel_pgtable_work.work);
> +	/*
> +	 * The workqueue may not exist yet while the system is booting.
> +	 * kernel_pgtable_drain_early() schedules the work once it does.
> +	 */
> +	if (system_state != SYSTEM_BOOTING)
> +		schedule_kernel_pgtable_free();
> +}
> +
> +static int __init kernel_pgtable_drain_early(void)
> +{
> +	/* Free the kernel page tables queued while booting. */
> +	schedule_kernel_pgtable_free();
> +	return 0;
>  }
> +core_initcall(kernel_pgtable_drain_early);
>  #endif
> --
> 2.55.0
>

--
Cheers, Lorenzo