Re: [PATCH v2 03/20] KVM: s390: Add gmap_helper_set_unused()

[Nina Schoetterl-Glausch <nsg@linux.ibm.com>]

On Wed, 2025-09-10 at 20:07 +0200, Claudio Imbrenda wrote:
> Add gmap_helper_set_unused() to mark userspace ptes as unused.
> 
> Core mm code will use that information to discard unused pages instead
> of attempting to swap them.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

LGTM
> ---
>  arch/s390/include/asm/gmap_helpers.h |  1 +
>  arch/s390/mm/gmap_helpers.c          | 64 ++++++++++++++++++++++++++++
>  2 files changed, 65 insertions(+)
> 
> diff --git a/arch/s390/include/asm/gmap_helpers.h b/arch/s390/include/asm/gmap_helpers.h
> index 5356446a61c4..459bd39d0887 100644
> --- a/arch/s390/include/asm/gmap_helpers.h
> +++ b/arch/s390/include/asm/gmap_helpers.h
> @@ -11,5 +11,6 @@
>  void gmap_helper_zap_one_page(struct mm_struct *mm, unsigned long vmaddr);
>  void gmap_helper_discard(struct mm_struct *mm, unsigned long vmaddr, unsigned long end);
>  int gmap_helper_disable_cow_sharing(void);
> +void gmap_helper_set_unused(struct mm_struct *mm, unsigned long vmaddr);
>  
>  #endif /* _ASM_S390_GMAP_HELPERS_H */
> diff --git a/arch/s390/mm/gmap_helpers.c b/arch/s390/mm/gmap_helpers.c
> index a45d417ad951..69ffc0c6b654 100644
> --- a/arch/s390/mm/gmap_helpers.c
> +++ b/arch/s390/mm/gmap_helpers.c
> @@ -91,6 +91,70 @@ void gmap_helper_discard(struct mm_struct *mm, unsigned long vmaddr, unsigned lo
>  }
>  EXPORT_SYMBOL_GPL(gmap_helper_discard);
>  
> +/**
> + * gmap_helper_set_unused() - mark a pte entry as unused
> + * @mm: the mm
> + * @vmaddr: the userspace address whose pte is to be marked
> + *
> + * Mark the pte corresponding the given address as unused. This will cause
> + * core mm code to just drop this page instead of swapping it.
> + *
> + * This function needs to be called with interrupts disabled (for example
> + * while holding a spinlock), or while holding the mmap lock. Normally this
> + * function is called as a result of an unmap operation, and thus KVM common
> + * code will already hold kvm->mmu_lock in write mode.
> + *
> + * Context: Needs to be called while holding the mmap lock or with interrupts
> + *          disabled.
> + */
> +void gmap_helper_set_unused(struct mm_struct *mm, unsigned long vmaddr)

Can you give this a better name? E.g. gmap_helper_try_set_pte_unused

> +{
> +	pmd_t *pmdp, pmd, pmdval;
> +	pud_t *pudp, pud;
> +	p4d_t *p4dp, p4d;
> +	pgd_t *pgdp, pgd;
> +	spinlock_t *ptl;
> +	pte_t *ptep;
> +
> +	pgdp = pgd_offset(mm, vmaddr);
> +	pgd = pgdp_get(pgdp);
> +	if (pgd_none(pgd) || !pgd_present(pgd))
> +		return;
> +
> +	p4dp = p4d_offset(pgdp, vmaddr);
> +	p4d = p4dp_get(p4dp);
> +	if (p4d_none(p4d) || !p4d_present(p4d))
> +		return;
> +
> +	pudp = pud_offset(p4dp, vmaddr);
> +	pud = pudp_get(pudp);
> +	if (pud_none(pud) || pud_leaf(pud) || !pud_present(pud))
> +		return;
> +
> +	pmdp = pmd_offset(pudp, vmaddr);
> +	pmd = pmdp_get_lockless(pmdp);
> +	if (pmd_none(pmd) || pmd_leaf(pmd) || !pmd_present(pmd))
> +		return;
> +
> +	ptep = pte_offset_map_rw_nolock(mm, pmdp, vmaddr, &pmdval, &ptl);
> +	if (!ptep)
> +		return;
> +
> +	if (spin_trylock(ptl)) {

Missing the comment you promised :) about deadlock prevention.

> +		/*
> +		 * Make sure the pte we are touching is still the correct
> +		 * one. In theory this check should not be needed, but

Why should it not be needed? I.e. why should we be protected against modification?
> +		 * better safe than sorry.
> +		 */
> +		if (likely(pmd_same(pmdval, pmdp_get_lockless(pmdp))))
> +			__atomic64_or(_PAGE_UNUSED, (long *)ptep);
> +		spin_unlock(ptl);
> +	}
> +
> +	pte_unmap(ptep);
> +}
> +EXPORT_SYMBOL_GPL(gmap_helper_set_unused);
> +
>  static int find_zeropage_pte_entry(pte_t *pte, unsigned long addr,
>  				   unsigned long end, struct mm_walk *walk)
>  {

-- 
IBM Deutschland Research & Development GmbH
Vorsitzender des Aufsichtsrats: Wolfgang Wendt
Geschäftsführung: David Faller
Sitz der Gesellschaft: Böblingen / Registergericht: Amtsgericht Stuttgart, HRB 243294