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

David Hildenbrand (Red Hat)
2026-01-18T11:23:20Z

On 12/17/25 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.

This patch seems to make s390x compilations sometimes unhappy:

Unverified Warning (likely false positive, kindly check if interested):

mm/memory.c:1911 zap_pte_range() error: uninitialized symbol 'pmdval'.

Warning ids grouped by kconfigs:

recent_errors
`-- s390-randconfig-r072-20260117
`-- mm-memory.c-zap_pte_range()-error:uninitialized-symbol-pmdval-.

I assume the compiler is not able to figure out that only when
try_get_and_clear_pmd() returns false that pmdval could be uninitialized.

Maybe it has to do with LTO?

After all, that function resides in a different compilation unit.

Which makes me wonder whether we want to just move try_get_and_clear_pmd()
and reclaim_pt_is_enabled() to internal.h or even just memory.c?

But then, maybe we could remove pt_reclaim.c completely and just have
try_to_free_pte() in memory.c as well?

I would just do the following cleanup:

From cfe97092f71fcc88f729f07ee0bc6816e3e398f0 Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Red Hat)" <david@kernel.org>
Date: Sun, 18 Jan 2026 12:20:55 +0100
Subject: [PATCH] mm: move pte table reclaim code to memory.c

Let's move the code and clean it up a bit along the way.

Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
---
MAINTAINERS | 1 -
mm/internal.h | 18 -------------
mm/memory.c | 70 ++++++++++++++++++++++++++++++++++++++++++-----
mm/pt_reclaim.c | 72 -------------------------------------------------
4 files changed, 64 insertions(+), 97 deletions(-)
delete mode 100644 mm/pt_reclaim.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 11720728d92f2..28e8e28bca3e5 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -16692,7 +16692,6 @@ R: Shakeel Butt <shakeel.butt@linux.dev>
R: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
L: linux-mm@kvack.org
S: Maintained
-F: mm/pt_reclaim.c
F: mm/vmscan.c
F: mm/workingset.c

diff --git a/mm/internal.h b/mm/internal.h
index 9508dbaf47cd4..ef71a1d9991f2 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1745,24 +1745,6 @@ int walk_page_range_debug(struct mm_struct *mm, unsigned long start,
unsigned long end, const struct mm_walk_ops *ops,
pgd_t *pgd, void *private);

-/* pt_reclaim.c */
-bool try_get_and_clear_pmd(struct mm_struct *mm, pmd_t *pmd, pmd_t *pmdval);
-void free_pte(struct mm_struct *mm, unsigned long addr, struct mmu_gather *tlb,
- pmd_t pmdval);
-void try_to_free_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr,
- struct mmu_gather *tlb);
-
-#ifdef CONFIG_PT_RECLAIM
-bool reclaim_pt_is_enabled(unsigned long start, unsigned long end,
- struct zap_details *details);
-#else
-static inline bool reclaim_pt_is_enabled(unsigned long start, unsigned long end,
- struct zap_details *details)
-{
- return false;
-}
-#endif /* CONFIG_PT_RECLAIM */
-
void dup_mm_exe_file(struct mm_struct *mm, struct mm_struct *oldmm);
int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm);

diff --git a/mm/memory.c b/mm/memory.c
index f2e9e05388743..a09226761a07f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1824,11 +1824,68 @@ static inline int do_zap_pte_range(struct mmu_gather *tlb,
return nr;
}

+static bool pte_table_reclaim_enabled(unsigned long start, unsigned long end,
+ struct zap_details *details)
+{
+ if (!IS_ENABLED(CONFIG_PT_RECLAIM))
+ return false;
+ return details && details->reclaim_pt && (end - start >= PMD_SIZE);
+}
+
+static bool zap_empty_pte_table(struct mm_struct *mm, pmd_t *pmd, pmd_t *pmdval)
+{
+ spinlock_t *pml = pmd_lockptr(mm, pmd);
+
+ if (!spin_trylock(pml))
+ return false;
+
+ *pmdval = pmdp_get_lockless(pmd);
+ pmd_clear(pmd);
+ spin_unlock(pml);
+
+ return true;
+}
+
+static bool zap_pte_table_if_empty(struct mm_struct *mm, pmd_t *pmd,
+ unsigned long addr, pmd_t *pmdval)
+{
+ spinlock_t *pml, *ptl = NULL;
+ pte_t *start_pte, *pte;
+ int i;
+
+ pml = pmd_lock(mm, pmd);
+ start_pte = pte_offset_map_rw_nolock(mm, pmd, addr, pmdval, &ptl);
+ if (!start_pte)
+ goto out_ptl;
+ if (ptl != pml)
+ spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
+
+ for (i = 0, pte = start_pte; i < PTRS_PER_PTE; i++, pte++) {
+ if (!pte_none(ptep_get(pte)))
+ goto out_ptl;
+ }
+ pte_unmap(start_pte);
+
+ pmd_clear(pmd);
+
+ if (ptl != pml)
+ spin_unlock(ptl);
+ spin_unlock(pml);
+ return true;
+out_ptl:
+ if (start_pte)
+ pte_unmap_unlock(start_pte, ptl);
+ if (ptl != pml)
+ spin_unlock(pml);
+ return false;
+}
+
static unsigned long zap_pte_range(struct mmu_gather *tlb,
struct vm_area_struct *vma, pmd_t *pmd,
unsigned long addr, unsigned long end,
struct zap_details *details)
{
+ bool can_reclaim_pt = pte_table_reclaim_enabled(addr, end, details);
bool force_flush = false, force_break = false;
struct mm_struct *mm = tlb->mm;
int rss[NR_MM_COUNTERS];
@@ -1837,7 +1894,6 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
pte_t *pte;
pmd_t pmdval;
unsigned long start = addr;
- bool can_reclaim_pt = reclaim_pt_is_enabled(start, end, details);
bool direct_reclaim = true;
int nr;

@@ -1878,7 +1934,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
* from being repopulated by another thread.
*/
if (can_reclaim_pt && direct_reclaim && addr == end)
- direct_reclaim = try_get_and_clear_pmd(mm, pmd, &pmdval);
+ direct_reclaim = zap_empty_pte_table(mm, pmd, &pmdval);

add_mm_rss_vec(mm, rss);
lazy_mmu_mode_disable();
@@ -1907,10 +1963,12 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
}

if (can_reclaim_pt) {
- if (direct_reclaim)
- free_pte(mm, start, tlb, pmdval);
- else
- try_to_free_pte(mm, pmd, start, tlb);
+ if (!direct_reclaim)
+ direct_reclaim = zap_pte_table_if_empty(mm, pmd, start, &pmdval);
+ if (direct_reclaim) {
+ pte_free_tlb(tlb, pmd_pgtable(pmdval), addr);
+ mm_dec_nr_ptes(mm);
+ }
}

return addr;
diff --git a/mm/pt_reclaim.c b/mm/pt_reclaim.c
deleted file mode 100644
index 46771cfff8239..0000000000000
--- a/mm/pt_reclaim.c
+++ /dev/null
@@ -1,72 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0
-#include <linux/hugetlb.h>
-#include <linux/pgalloc.h>
-
-#include <asm/tlb.h>
-
-#include "internal.h"
-
-bool reclaim_pt_is_enabled(unsigned long start, unsigned long end,
- struct zap_details *details)
-{
- return details && details->reclaim_pt && (end - start >= PMD_SIZE);
-}
-
-bool try_get_and_clear_pmd(struct mm_struct *mm, pmd_t *pmd, pmd_t *pmdval)
-{
- spinlock_t *pml = pmd_lockptr(mm, pmd);
-
- if (!spin_trylock(pml))
- return false;
-
- *pmdval = pmdp_get_lockless(pmd);
- pmd_clear(pmd);
- spin_unlock(pml);
-
- return true;
-}
-
-void free_pte(struct mm_struct *mm, unsigned long addr, struct mmu_gather *tlb,
- pmd_t pmdval)
-{
- pte_free_tlb(tlb, pmd_pgtable(pmdval), addr);
- mm_dec_nr_ptes(mm);
-}
-
-void try_to_free_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr,
- struct mmu_gather *tlb)
-{
- pmd_t pmdval;
- spinlock_t *pml, *ptl = NULL;
- pte_t *start_pte, *pte;
- int i;
-
- pml = pmd_lock(mm, pmd);
- start_pte = pte_offset_map_rw_nolock(mm, pmd, addr, &pmdval, &ptl);
- if (!start_pte)
- goto out_ptl;
- if (ptl != pml)
- spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
-
- /* Check if it is empty PTE page */
- for (i = 0, pte = start_pte; i < PTRS_PER_PTE; i++, pte++) {
- if (!pte_none(ptep_get(pte)))
- goto out_ptl;
- }
- pte_unmap(start_pte);
-
- pmd_clear(pmd);
-
- if (ptl != pml)
- spin_unlock(ptl);
- spin_unlock(pml);
-
- free_pte(mm, addr, tlb, pmdval);
-
- return;
-out_ptl:
- if (start_pte)
- pte_unmap_unlock(start_pte, ptl);
- if (ptl != pml)
- spin_unlock(pml);
-}
--
2.52.0

Completely untested, of course.

--
Cheers

David