linux-kernel - Re: [PATCH v3 7/7] mm: make PT_RECLAIM depends on MMU_GATHER_RCU_TABLE

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Message-ID: <b95f042f-8fc8-4b6c-b9db-b198efdd0973@kernel.org>
Date: Tue, 27 Jan 2026 12:29:55 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Qi Zheng <qi.zheng@...ux.dev>, Andreas Larsson <andreas@...sler.com>
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-alpha@...r.kernel.org, loongarch@...ts.linux.dev,
 linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
 linux-um@...ts.infradead.org, Qi Zheng <zhengqi.arch@...edance.com>,
 sparclinux <sparclinux@...r.kernel.org>, will@...nel.org,
 peterz@...radead.org, akpm@...ux-foundation.org, aneesh.kumar@...nel.org,
 npiggin@...il.com, dev.jain@....com, ioworker0@...il.com, linmag7@...il.com
Subject: Re: [PATCH v3 7/7] mm: make PT_RECLAIM depends on
 MMU_GATHER_RCU_TABLE_FREE

On 1/26/26 07:59, Qi Zheng wrote:
> 
> 
> On 1/23/26 11:15 PM, Andreas Larsson wrote:
>> On 2025-12-17 10:45, Qi Zheng wrote:
>>> From: Qi Zheng <zhengqi.arch@...edance.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@...edance.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:

Can you elaborate, I don't understand what you mean :)

Is it also a problem on architectures like s390x and ppc, where we 
squeeze multiple page tables into a physical pages?

> 
> __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?

Or we invert it and only disable it for the known-problematic architectures?

-- 
Cheers

David