Re: [PATCH 1/8] mm: Add ptep_try_set() for lockless empty-slot installs

[Andrea Righi <arighi@nvidia.com>]

Hi Tejun,

On Wed, May 20, 2026 at 01:50:45PM -1000, Tejun Heo wrote:
> Add ptep_try_set(ptep, new_pte): atomically set *ptep to new_pte iff it is
> currently pte_none(). Returns true on success, false if the slot was already
> populated or the arch has no implementation.
> 
> The intended caller is the upcoming bpf_arena kernel-side fault recovery
> path. The install runs from a page fault that can be nested under locks
> held by the faulting kernel caller (e.g. a BPF program holding
> raw_res_spin_lock_irqsave on its arena's spinlock), so trylock-and-retry
> would A-A deadlock. Lock-free cmpxchg is the only viable option, which
> constrains this helper to special kernel page tables where concurrent
> writers cooperate via atomic accessors.
> 
> The generic version in <linux/pgtable.h> returns false. x86 and arm64
> override with try_cmpxchg-based implementations on the underlying pteval.
> Other architectures get the false stub - the callers there already fall
> through to oops.
> 
> v2: Rename to ptep_try_set(). Tighten kerneldoc for kernel-PTE use.
>     (David, Alexei)
> 
> Suggested-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Suggested-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: David Hildenbrand <david@kernel.org>
> ---
>  arch/arm64/include/asm/pgtable.h |  8 ++++++++
>  arch/x86/include/asm/pgtable.h   |  8 ++++++++
>  include/linux/pgtable.h          | 26 ++++++++++++++++++++++++++
>  3 files changed, 42 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 9029b81ccbe8..a129be91ef2c 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1830,6 +1830,14 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>  	return __ptep_get_and_clear(mm, addr, ptep);
>  }
>  
> +static inline bool ptep_try_set(pte_t *ptep, pte_t new_pte)
> +{
> +	pteval_t old = 0;
> +
> +	return try_cmpxchg(&pte_val(*ptep), &old, pte_val(new_pte));
> +}
> +#define ptep_try_set ptep_try_set
> +
>  #define test_and_clear_young_ptes test_and_clear_young_ptes
>  static inline bool test_and_clear_young_ptes(struct vm_area_struct *vma,
>  		unsigned long addr, pte_t *ptep, unsigned int nr)
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 13e3e9a054cb..047e273a4eab 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1284,6 +1284,14 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm,
>  	} while (!try_cmpxchg((long *)&ptep->pte, (long *)&old_pte, *(long *)&new_pte));
>  }
>  
> +static inline bool ptep_try_set(pte_t *ptep, pte_t new_pte)
> +{
> +	pte_t old_pte = __pte(0);
> +
> +	return try_cmpxchg((long *)&ptep->pte, (long *)&old_pte, *(long *)&new_pte);
> +}

Minor nit (feel free to ignore), on x86 pte_none() is defined as:

static inline int pte_none(pte_t pte)
{
	return !(pte.pte & ~(_PAGE_KNL_ERRATUM_MASK));
}

With:

#if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
#define _PAGE_KNL_ERRATUM_MASK (_PAGE_DIRTY | _PAGE_ACCESSED)
#else
#define _PAGE_KNL_ERRATUM_MASK 0
#endif

If that mask has the D/A bits set, try_cmpxchg(..., &old=0, ...) will reject a
PTE that has only those bits set, even though pte_none() would return true. I
think this is fine for the bpf_arena use case, since hardware shouldn't set A/D
for fresh pages that the BPF prog hasn't touched.

Maybe it's worth adding a comment (something along these lines)?

 /*
  * Note: strictly-zero compare is narrower than pte_none() (see pte_none() and
  * _PAGE_KNL_ERRATUM_MASK), but the gap is harmless in practice: HW shouldn't
  * set _PAGE_DIRTY | _PAGE_ACCESSED bits on entries the caller never touched.
  */

Other than that, looks good to me.

Reviewed-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> +#define ptep_try_set ptep_try_set
> +
>  #define flush_tlb_fix_spurious_fault(vma, address, ptep) do { } while (0)
>  
>  #define  __HAVE_ARCH_PMDP_SET_ACCESS_FLAGS
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index cdd68ed3ae1a..d68374f404c1 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1036,6 +1036,32 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
>  }
>  #endif
>  
> +#ifndef ptep_try_set
> +/**
> + * ptep_try_set - atomically set an empty kernel PTE
> + * @ptep: page table entry
> + * @new_pte: value to install
> + *
> + * Atomically set *@ptep to @new_pte iff *@ptep is pte_none(). Return
> + * true on success, false if the slot was already populated or the
> + * arch has no implementation.
> + *
> + * For special kernel page tables only - never user page tables. The
> + * caller must prevent concurrent teardown of @ptep and must accept
> + * that other writers may race. Concurrent clearers must use
> + * ptep_get_and_clear() so racing accesses agree on the outcome.
> + *
> + * Architectures opt in by providing a cmpxchg-based override and
> + * defining ptep_try_set as an identity macro. The generic stub
> + * returns false, which is correct for callers that fall through to
> + * oops on failure.
> + */
> +static inline bool ptep_try_set(pte_t *ptep, pte_t new_pte)
> +{
> +	return false;
> +}
> +#endif
> +
>  #ifndef wrprotect_ptes
>  /**
>   * wrprotect_ptes - Write-protect PTEs that map consecutive pages of the same
> -- 
> 2.54.0
>