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

[Qi Zheng <qi.zheng@linux.dev>]

On 1/23/26 11:15 PM, Andreas Larsson wrote:
> 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

Thanks for your report!

On sparc64, pmd and pud levels are not of struct page:

__pmd_free_tlb/__pud_free_tlb
--> pgtable_free_tlb(tlb, pud/pmd, false). <=== is_page == false
     --> tlb_remove_table

So in __tlb_remove_table_one(), the table cannot be treated as
ptdesc because it does not have an pt_rcu_head member.

Hi David, it seems we still need to keep ARCH_SUPPORTS_PT_RECLAIM?

Thanks,
Qi

> ...
> 
> 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
>