[PATCH v3 0/7] enable PT_RECLAIM on all 64-bit architectures

[PATCH v3 0/7] enable PT_RECLAIM on all 64-bit architectures

[Qi Zheng]

From: Qi Zheng <zhengqi.arch@bytedance.com>

Changes in v3:
 - modify the commit message in [PATCH v3 1/7] (suggested by David Hildenbrand)
 - make PT_RECLAIM depends on MMU_GATHER_RCU_TABLE_FREE instead of 64BIT
 - collect Acked-by
 - rebase onto the next-20251217

Changelog in v2:
 - fix compilation errors (reported by Magnus Lindholm and kernel test robot)
 - adjust some code style (suggested by Huacai Chen)
 - make PT_RECLAIM user-unselectable (suggested by David Hildenbrand)
 - rebase onto the next-20251119

Hi all,

This series aims to enable PT_RECLAIM on all 64-bit architectures.

On a 64-bit system, madvise(MADV_DONTNEED) may cause a large number of empty PTE
page table pages (such as 100GB+). To resolve this problem, we need to enable
PT_RECLAIM, which depends on MMU_GATHER_RCU_TABLE_FREE.

Therefore, this series first enables MMU_GATHER_RCU_TABLE_FREE on all 64-bit
architectures, and finally makes PT_RECLAIM depend on MMU_GATHER_RCU_TABLE_FREE.
This way, PT_RECLAIM can be enabled by default on all 64-bit architectures.

Of course, this will also be enabled on some 32-bit architectures that already
support MMU_GATHER_RCU_TABLE_FREE. That's fine, PT_RECLAIM works well on all
32-bit architectures as well. Although the benefit isn't significant, there's
still memory that can be reclaimed. Perhaps PT_RECLAIM can be enabled on all
32-bit architectures in the future.

Comments and suggestions are welcome!

Thanks,
Qi

Qi Zheng (7):
  mm: change mm/pt_reclaim.c to use asm/tlb.h instead of
    asm-generic/tlb.h
  alpha: mm: enable MMU_GATHER_RCU_TABLE_FREE
  LoongArch: mm: enable MMU_GATHER_RCU_TABLE_FREE
  mips: mm: enable MMU_GATHER_RCU_TABLE_FREE
  parisc: mm: enable MMU_GATHER_RCU_TABLE_FREE
  um: mm: enable MMU_GATHER_RCU_TABLE_FREE
  mm: make PT_RECLAIM depends on MMU_GATHER_RCU_TABLE_FREE

 arch/alpha/Kconfig                   | 1 +
 arch/alpha/include/asm/tlb.h         | 6 +++---
 arch/loongarch/Kconfig               | 1 +
 arch/loongarch/include/asm/pgalloc.h | 7 +++----
 arch/mips/Kconfig                    | 1 +
 arch/mips/include/asm/pgalloc.h      | 7 +++----
 arch/parisc/Kconfig                  | 1 +
 arch/parisc/include/asm/tlb.h        | 4 ++--
 arch/um/Kconfig                      | 1 +
 arch/x86/Kconfig                     | 1 -
 mm/Kconfig                           | 9 ++-------
 mm/pt_reclaim.c                      | 2 +-
 12 files changed, 19 insertions(+), 22 deletions(-)

-- 
2.20.1

[PATCH v3 1/7] mm: change mm/pt_reclaim.c to use asm/tlb.h instead of asm-generic/tlb.h

[Qi Zheng]

From: Qi Zheng <zhengqi.arch@bytedance.com>

Generally, the asm/tlb.h will include asm-generic/tlb.h, so change
mm/pt_reclaim.c to use asm/tlb.h instead of asm-generic/tlb.h. This is a
preparation for enabling CONFIG_PT_RECLAIM on other architectures, such as
alpha.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
---
 mm/pt_reclaim.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/pt_reclaim.c b/mm/pt_reclaim.c
index 0d9cfbf4fe5d8..46771cfff8239 100644
--- a/mm/pt_reclaim.c
+++ b/mm/pt_reclaim.c
@@ -2,7 +2,7 @@
 #include <linux/hugetlb.h>
 #include <linux/pgalloc.h>
 
-#include <asm-generic/tlb.h>
+#include <asm/tlb.h>
 
 #include "internal.h"
 
-- 
2.20.1

[PATCH v3 2/7] alpha: mm: enable MMU_GATHER_RCU_TABLE_FREE

[Qi Zheng]

From: Qi Zheng <zhengqi.arch@bytedance.com>

On a 64-bit system, madvise(MADV_DONTNEED) may cause a large number of
empty PTE page table pages (such as 100GB+). To resolve this problem,
first enable MMU_GATHER_RCU_TABLE_FREE to prepare for enabling the
PT_RECLAIM feature, which resolves this problem.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Matt Turner <mattst88@gmail.com>
---
 arch/alpha/Kconfig           | 1 +
 arch/alpha/include/asm/tlb.h | 6 +++---
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index 80367f2cf821c..6c7dbf0adad62 100644
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -38,6 +38,7 @@ config ALPHA
 	select OLD_SIGSUSPEND
 	select CPU_NO_EFFICIENT_FFS if !ALPHA_EV67
 	select MMU_GATHER_NO_RANGE
+	select MMU_GATHER_RCU_TABLE_FREE
 	select SPARSEMEM_EXTREME if SPARSEMEM
 	select ZONE_DMA
 	help
diff --git a/arch/alpha/include/asm/tlb.h b/arch/alpha/include/asm/tlb.h
index 4f79e331af5ea..ad586b898fd6b 100644
--- a/arch/alpha/include/asm/tlb.h
+++ b/arch/alpha/include/asm/tlb.h
@@ -4,7 +4,7 @@
 
 #include <asm-generic/tlb.h>
 
-#define __pte_free_tlb(tlb, pte, address)		pte_free((tlb)->mm, pte)
-#define __pmd_free_tlb(tlb, pmd, address)		pmd_free((tlb)->mm, pmd)
- 
+#define __pte_free_tlb(tlb, pte, address)	tlb_remove_ptdesc((tlb), page_ptdesc(pte))
+#define __pmd_free_tlb(tlb, pmd, address)	tlb_remove_ptdesc((tlb), virt_to_ptdesc(pmd))
+
 #endif
-- 
2.20.1

[PATCH v3 3/7] LoongArch: mm: enable MMU_GATHER_RCU_TABLE_FREE

[Qi Zheng]

From: Qi Zheng <zhengqi.arch@bytedance.com>

On a 64-bit system, madvise(MADV_DONTNEED) may cause a large number of
empty PTE page table pages (such as 100GB+). To resolve this problem,
first enable MMU_GATHER_RCU_TABLE_FREE to prepare for enabling the
PT_RECLAIM feature, which resolves this problem.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: WANG Xuerui <kernel@xen0n.name>
---
 arch/loongarch/Kconfig               | 1 +
 arch/loongarch/include/asm/pgalloc.h | 7 +++----
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index 730f342145197..43d5b863e1fb2 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -187,6 +187,7 @@ config LOONGARCH
 	select IRQ_LOONGARCH_CPU
 	select LOCK_MM_AND_FIND_VMA
 	select MMU_GATHER_MERGE_VMAS if MMU
+	select MMU_GATHER_RCU_TABLE_FREE
 	select MODULES_USE_ELF_RELA if MODULES
 	select NEED_PER_CPU_EMBED_FIRST_CHUNK
 	select NEED_PER_CPU_PAGE_FIRST_CHUNK
diff --git a/arch/loongarch/include/asm/pgalloc.h b/arch/loongarch/include/asm/pgalloc.h
index 08dcc698ec184..248f62d0b590e 100644
--- a/arch/loongarch/include/asm/pgalloc.h
+++ b/arch/loongarch/include/asm/pgalloc.h
@@ -55,8 +55,7 @@ static inline pte_t *pte_alloc_one_kernel(struct mm_struct *mm)
 	return pte;
 }
 
-#define __pte_free_tlb(tlb, pte, address)	\
-	tlb_remove_ptdesc((tlb), page_ptdesc(pte))
+#define __pte_free_tlb(tlb, pte, address)	tlb_remove_ptdesc((tlb), page_ptdesc(pte))
 
 #ifndef __PAGETABLE_PMD_FOLDED
 
@@ -79,7 +78,7 @@ static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long address)
 	return pmd;
 }
 
-#define __pmd_free_tlb(tlb, x, addr)	pmd_free((tlb)->mm, x)
+#define __pmd_free_tlb(tlb, x, addr)	tlb_remove_ptdesc((tlb), virt_to_ptdesc(x))
 
 #endif
 
@@ -99,7 +98,7 @@ static inline pud_t *pud_alloc_one(struct mm_struct *mm, unsigned long address)
 	return pud;
 }
 
-#define __pud_free_tlb(tlb, x, addr)	pud_free((tlb)->mm, x)
+#define __pud_free_tlb(tlb, x, addr)	tlb_remove_ptdesc((tlb), virt_to_ptdesc(x))
 
 #endif /* __PAGETABLE_PUD_FOLDED */
 
-- 
2.20.1

[PATCH v3 4/7] mips: mm: enable MMU_GATHER_RCU_TABLE_FREE

[Qi Zheng]

From: Qi Zheng <zhengqi.arch@bytedance.com>

On a 64-bit system, madvise(MADV_DONTNEED) may cause a large number of
empty PTE page table pages (such as 100GB+). To resolve this problem,
first enable MMU_GATHER_RCU_TABLE_FREE to prepare for enabling the
PT_RECLAIM feature, which resolves this problem.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
---
 arch/mips/Kconfig               | 1 +
 arch/mips/include/asm/pgalloc.h | 7 +++----
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index b88b97139fa8e..c0c94e26ce396 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -99,6 +99,7 @@ config MIPS
 	select IRQ_FORCED_THREADING
 	select ISA if EISA
 	select LOCK_MM_AND_FIND_VMA
+	select MMU_GATHER_RCU_TABLE_FREE
 	select MODULES_USE_ELF_REL if MODULES
 	select MODULES_USE_ELF_RELA if MODULES && 64BIT
 	select PERF_USE_VMALLOC
diff --git a/arch/mips/include/asm/pgalloc.h b/arch/mips/include/asm/pgalloc.h
index 7a04381efa0b5..895bf79e76762 100644
--- a/arch/mips/include/asm/pgalloc.h
+++ b/arch/mips/include/asm/pgalloc.h
@@ -48,8 +48,7 @@ static inline void pud_populate(struct mm_struct *mm, pud_t *pud, pmd_t *pmd)
 extern void pgd_init(void *addr);
 extern pgd_t *pgd_alloc(struct mm_struct *mm);
 
-#define __pte_free_tlb(tlb, pte, address)	\
-	tlb_remove_ptdesc((tlb), page_ptdesc(pte))
+#define __pte_free_tlb(tlb, pte, address)	tlb_remove_ptdesc((tlb), page_ptdesc(pte))
 
 #ifndef __PAGETABLE_PMD_FOLDED
 
@@ -72,7 +71,7 @@ static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long address)
 	return pmd;
 }
 
-#define __pmd_free_tlb(tlb, x, addr)	pmd_free((tlb)->mm, x)
+#define __pmd_free_tlb(tlb, x, addr)	tlb_remove_ptdesc((tlb), virt_to_ptdesc(x))
 
 #endif
 
@@ -97,7 +96,7 @@ static inline void p4d_populate(struct mm_struct *mm, p4d_t *p4d, pud_t *pud)
 	set_p4d(p4d, __p4d((unsigned long)pud));
 }
 
-#define __pud_free_tlb(tlb, x, addr)	pud_free((tlb)->mm, x)
+#define __pud_free_tlb(tlb, x, addr)	tlb_remove_ptdesc((tlb), virt_to_ptdesc(x))
 
 #endif /* __PAGETABLE_PUD_FOLDED */
 
-- 
2.20.1

[PATCH v3 5/7] parisc: mm: enable MMU_GATHER_RCU_TABLE_FREE

[Qi Zheng]

From: Qi Zheng <zhengqi.arch@bytedance.com>

On a 64-bit system, madvise(MADV_DONTNEED) may cause a large number of
empty PTE page table pages (such as 100GB+). To resolve this problem,
first enable MMU_GATHER_RCU_TABLE_FREE to prepare for enabling the
PT_RECLAIM feature, which resolves this problem.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
---
 arch/parisc/Kconfig           | 1 +
 arch/parisc/include/asm/tlb.h | 4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
index 47fd9662d8005..62d5a89d5c7bc 100644
--- a/arch/parisc/Kconfig
+++ b/arch/parisc/Kconfig
@@ -79,6 +79,7 @@ config PARISC
 	select GENERIC_CLOCKEVENTS
 	select CPU_NO_EFFICIENT_FFS
 	select THREAD_INFO_IN_TASK
+	select MMU_GATHER_RCU_TABLE_FREE
 	select NEED_DMA_MAP_STATE
 	select NEED_SG_DMA_LENGTH
 	select HAVE_ARCH_KGDB
diff --git a/arch/parisc/include/asm/tlb.h b/arch/parisc/include/asm/tlb.h
index 44235f367674d..4501fee0a8fa4 100644
--- a/arch/parisc/include/asm/tlb.h
+++ b/arch/parisc/include/asm/tlb.h
@@ -5,8 +5,8 @@
 #include <asm-generic/tlb.h>
 
 #if CONFIG_PGTABLE_LEVELS == 3
-#define __pmd_free_tlb(tlb, pmd, addr)	pmd_free((tlb)->mm, pmd)
+#define __pmd_free_tlb(tlb, pmd, addr)	tlb_remove_ptdesc((tlb), virt_to_ptdesc(pmd))
 #endif
-#define __pte_free_tlb(tlb, pte, addr)	pte_free((tlb)->mm, pte)
+#define __pte_free_tlb(tlb, pte, addr)	tlb_remove_ptdesc((tlb), page_ptdesc(pte))
 
 #endif
-- 
2.20.1

[PATCH v3 6/7] um: mm: enable MMU_GATHER_RCU_TABLE_FREE

[Qi Zheng]

From: Qi Zheng <zhengqi.arch@bytedance.com>

On a 64-bit system, madvise(MADV_DONTNEED) may cause a large number of
empty PTE page table pages (such as 100GB+). To resolve this problem,
first enable MMU_GATHER_RCU_TABLE_FREE to prepare for enabling the
PT_RECLAIM feature, which resolves this problem.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
---
 arch/um/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/um/Kconfig b/arch/um/Kconfig
index 8415d39b0d430..098cda44db225 100644
--- a/arch/um/Kconfig
+++ b/arch/um/Kconfig
@@ -42,6 +42,7 @@ config UML
 	select HAVE_SYSCALL_TRACEPOINTS
 	select THREAD_INFO_IN_TASK
 	select SPARSE_IRQ
+	select MMU_GATHER_RCU_TABLE_FREE
 
 config MMU
 	bool
-- 
2.20.1

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

[Qi Zheng]

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.
-- 
2.20.1

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

[Wei Yang]

On Wed, Dec 17, 2025 at 05:45:48PM +0800, 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, Qi

I am new to PT_RECLAIM, when reading related code I got one question.

Before this patch,  we could have this config combination:

    CONFIG_MMU_GATHER_RCU_TABLE_FREE & !CONFIG_PT_RECLAIM

This means tlb_remove_table_free() is rcu version while tlb_remove_table_one()
is semi rcu version.

I am curious could we use rcu version tlb_remove_table_one() for this case?
Use rcu version tlb_remove_table_one() if CONFIG_MMU_GATHER_RCU_TABLE_FREE. Is
there some limitation here?

Thanks in advance for your explanation.


-- 
Wei Yang
Help you, Help me

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

[Qi Zheng]

On 12/31/25 5:42 PM, Wei Yang wrote:
> On Wed, Dec 17, 2025 at 05:45:48PM +0800, 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, Qi
> 
> I am new to PT_RECLAIM, when reading related code I got one question.
> 
> Before this patch,  we could have this config combination:
> 
>      CONFIG_MMU_GATHER_RCU_TABLE_FREE & !CONFIG_PT_RECLAIM
> 
> This means tlb_remove_table_free() is rcu version while tlb_remove_table_one()
> is semi rcu version.
> 
> I am curious could we use rcu version tlb_remove_table_one() for this case?
> Use rcu version tlb_remove_table_one() if CONFIG_MMU_GATHER_RCU_TABLE_FREE. Is
> there some limitation here?

I think there's no problem. The rcu version can also ensure that the
fast GUP works well.

> 
> Thanks in advance for your explanation.
> 
>