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

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

On 1/18/26 7:23 PM, David Hildenbrand (Red Hat) wrote:
> 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):

I believe it is a false positive.

> 
>      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

Make sense, and LGTM. The reason it was placed in mm/pt_reclaim.c before
was because there would be other paths calling these functions in the
future. However, it can be separated out or put into a header file when
there are actually such callers.

would you be willing to send out an official patch?

Thanks,
Qi

> 
> 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);
> -}