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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/17/26 23:12, Tejun Heo wrote:
> Add ptep_try_install(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 and may have to
> contend with locks already held by the faulting kernel caller, so
> keeping it lock-free via cmpxchg is the safe choice.
> 
> 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.
> 
> Suggested-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Suggested-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---

[...]

>  
> +#ifndef __HAVE_ARCH_PTEP_TRY_INSTALL
> +/**
> + * ptep_try_install - atomically install an empty 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. Architectures opt in by providing a cmpxchg-based
> + * override. The generic stub returns false.
> + */
> +static inline bool ptep_try_install(pte_t *ptep, pte_t new_pte)
> +{
> +	return false;
> +}
> +#endif

Ehm, what?

This is a very, very, very bad generic idea/interface.

On which ptes is this supposed to be used? User ptes or kernel ptes?

Surely we don't want this on user ptes.

-- 
Cheers,

David