Re: [PATCH v3 7/7] mm: make PT_RECLAIM depends on MMU_GATHER_RCU_TABLE_FREE

[Andreas Larsson <andreas@gaisler.com>]

On 2025-12-17 10:45, Qi Zheng wrote:
> From: Qi Zheng <zhengqi.arch@bytedance.com>
> 
> The PT_RECLAIM can work on all architectures that support
> MMU_GATHER_RCU_TABLE_FREE, so make PT_RECLAIM depends on
> MMU_GATHER_RCU_TABLE_FREE.
> 
> BTW, change PT_RECLAIM to be enabled by default, since nobody should want
> to turn it off.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>  arch/x86/Kconfig | 1 -
>  mm/Kconfig       | 9 ++-------
>  2 files changed, 2 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 80527299f859a..0d22da56a71b0 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -331,7 +331,6 @@ config X86
>  	select FUNCTION_ALIGNMENT_4B
>  	imply IMA_SECURE_AND_OR_TRUSTED_BOOT    if EFI
>  	select HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
> -	select ARCH_SUPPORTS_PT_RECLAIM		if X86_64
>  	select ARCH_SUPPORTS_SCHED_SMT		if SMP
>  	select SCHED_SMT			if SMP
>  	select ARCH_SUPPORTS_SCHED_CLUSTER	if SMP
> diff --git a/mm/Kconfig b/mm/Kconfig
> index bd0ea5454af82..fc00b429b7129 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1447,14 +1447,9 @@ config ARCH_HAS_USER_SHADOW_STACK
>  	  The architecture has hardware support for userspace shadow call
>            stacks (eg, x86 CET, arm64 GCS or RISC-V Zicfiss).
>  
> -config ARCH_SUPPORTS_PT_RECLAIM
> -	def_bool n
> -
>  config PT_RECLAIM
> -	bool "reclaim empty user page table pages"
> -	default y
> -	depends on ARCH_SUPPORTS_PT_RECLAIM && MMU && SMP
> -	select MMU_GATHER_RCU_TABLE_FREE
> +	def_bool y
> +	depends on MMU_GATHER_RCU_TABLE_FREE
>  	help
>  	  Try to reclaim empty user page table pages in paths other than munmap
>  	  and exit_mmap path.

Hi,

This patch unfortunately results in a WARN_ON_ONCE and unaligned
accesses on sparc64:

$ stress-ng --mmaphuge 20 -t 60
stress-ng: info:  [559] setting to a 1 min run per stressor
stress-ng: info:  [559] dispatching hogs: 20 mmaphuge
[  560.592569] ------------[ cut here ]------------
[  560.592663] WARNING: kernel/rcu/tree.c:3098 at __call_rcu_common.constprop.0+0x200/0x760, CPU#4: stress-ng-mmaph/568
[  560.592777] CPU: 4 UID: 1000 PID: 568 Comm: stress-ng-mmaph Not tainted 6.19.0-rc5-00127-g62fc9f6ccb97 #8 VOLUNTARY
[  560.592805] Call Trace:
[  560.592812] [<00000000004368b8>] dump_stack+0x8/0x60
[  560.592844] [<0000000000482a60>] __warn+0xe0/0x140
[  560.592878] [<0000000000482b64>] warn_slowpath_fmt+0xa4/0x120
[  560.592901] [<0000000000526a40>] __call_rcu_common.constprop.0+0x200/0x760
[  560.592931] [<0000000000526fd0>] call_rcu+0x10/0x20
[  560.592954] [<0000000000730838>] tlb_remove_table+0x98/0xc0
[  560.592986] [<000000000071bec4>] free_pgd_range+0x224/0x4c0
[  560.593021] [<000000000071c35c>] free_pgtables+0x1fc/0x240
[  560.593042] [<000000000074a6f0>] vms_clear_ptes+0x110/0x140
[  560.593068] [<000000000074c3dc>] vms_complete_munmap_vmas+0x5c/0x280
[  560.593094] [<000000000074de5c>] do_vmi_align_munmap+0x1dc/0x260
[  560.593117] [<000000000074df80>] do_vmi_munmap+0xa0/0x140
[  560.593142] [<000000000074fb2c>] __vm_munmap+0x8c/0x160
[  560.593168] [<000000000072cfd4>] vm_munmap+0x14/0x40
[  560.593190] [<00000000004402a8>] sys_64_munmap+0x88/0xa0
[  560.593221] [<0000000000406274>] linux_sparc_syscall+0x34/0x44
[  560.593274] ---[ end trace 0000000000000000 ]---
[  560.593960] log_unaligned: 209 callbacks suppressed
[  560.593979] Kernel unaligned access at TPC[526a4c] __call_rcu_common.constprop.0+0x20c/0x760
[  560.594121] Kernel unaligned access at TPC[526864] __call_rcu_common.constprop.0+0x24/0x760
[  560.594198] Kernel unaligned access at TPC[52b3c4] rcu_segcblist_enqueue+0x24/0x40
[  560.594275] Kernel unaligned access at TPC[526860] __call_rcu_common.constprop.0+0x20/0x760
[  560.594360] Kernel unaligned access at TPC[526864] __call_rcu_common.constprop.0+0x24/0x760
[  567.054127] log_unaligned: 1105 callbacks suppressed
[  567.054167] Kernel unaligned access at TPC[526860] __call_rcu_common.constprop.0+0x20/0x760
[  567.054331] Kernel unaligned access at TPC[526864] __call_rcu_common.constprop.0+0x24/0x760
[  567.054410] Kernel unaligned access at TPC[52b3c4] rcu_segcblist_enqueue+0x24/0x40
...

I bisected to this one on mm-unstable from approximately 2026-01-12.

The warning is from 

	/* Misaligned rcu_head! */
	WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));

in __call_rcu_common() and the unaligned accesses follows from there.

Regards,
Andreas