Re: [PATCH v1 3/3] mm/ksm: move disabling KSM from s390/gmap code to KSM code

Stefan Roesch
2023-04-20T21:42:39Z

David Hildenbrand <david@redhat.com> writes:

> Let's factor out actual disabling of KSM. The existing
> "mm->def_flags &= ~VM_MERGEABLE;" was essentially a NOP and can be dropped,
> because def_flags should never include VM_MERGEABLE. Note that we don't
> currently prevent re-enabling KSM.
>
> This should now be faster in case KSM was never enabled, because we only
> conditionally iterate all VMAs. Further, it certainly looks cleaner.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> arch/s390/mm/gmap.c | 20 +-------------------
> include/linux/ksm.h | 6 ++++++
> mm/ksm.c | 11 +++++++++++
> 3 files changed, 18 insertions(+), 19 deletions(-)
>
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index 0949811761e6..dfe905c7bd8e 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -2585,30 +2585,12 @@ EXPORT_SYMBOL_GPL(s390_enable_sie);
>
> int gmap_mark_unmergeable(void)
> {
> - struct mm_struct *mm = current->mm;
> - struct vm_area_struct *vma;
> - unsigned long vm_flags;
> - int ret;
> - VMA_ITERATOR(vmi, mm, 0);
> -
> /*
> * Make sure to disable KSM (if enabled for the whole process or
> * individual VMAs). Note that nothing currently hinders user space
> * from re-enabling it.
> */
> - clear_bit(MMF_VM_MERGE_ANY, &mm->flags);
> -
> - for_each_vma(vmi, vma) {
> - /* Copy vm_flags to avoid partial modifications in ksm_madvise */
> - vm_flags = vma->vm_flags;
> - ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> - MADV_UNMERGEABLE, &vm_flags);
> - if (ret)
> - return ret;
> - vm_flags_reset(vma, vm_flags);
> - }
> - mm->def_flags &= ~VM_MERGEABLE;
>

This clears the def_flags struct member, however, in ksm_disable() we
clear the __flags struct member. Is this a problem?

> - return 0;
> + return ksm_disable(current->mm);
> }
> EXPORT_SYMBOL_GPL(gmap_mark_unmergeable);
>
> diff --git a/include/linux/ksm.h b/include/linux/ksm.h
> index 7108bc65dc2a..b3d8b7849e18 100644
> --- a/include/linux/ksm.h
> +++ b/include/linux/ksm.h
> @@ -22,6 +22,7 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
> void ksm_add_vma(struct vm_area_struct *vma);
> int ksm_enable_merge_any(struct mm_struct *mm);
> int ksm_disable_merge_any(struct mm_struct *mm);
> +int ksm_disable(struct mm_struct *mm);
>
> int __ksm_enter(struct mm_struct *mm);
> void __ksm_exit(struct mm_struct *mm);
> @@ -75,6 +76,11 @@ static inline void ksm_add_vma(struct vm_area_struct *vma)
> {
> }
>
> +static inline int ksm_disable(struct mm_struct *mm)
> +{
> + return 0;
> +}
> +
> static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
> {
> return 0;
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 813f7fbc1832..208311cbb019 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2616,6 +2616,17 @@ int ksm_disable_merge_any(struct mm_struct *mm)
> return 0;
> }
>
> +int ksm_disable(struct mm_struct *mm)
> +{
> + mmap_assert_write_locked(mm);
> +
> + if (!test_bit(MMF_VM_MERGEABLE, &mm->flags))
> + return 0;
> + if (test_bit(MMF_VM_MERGE_ANY, &mm->flags))
> + return ksm_disable_merge_any(mm);
> + return ksm_del_vmas(mm);
> +}
> +
> int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
> unsigned long end, int advice, unsigned long *vm_flags)
> {