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From:   Stefan Roesch <shr@devkernel.io>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-s390@vger.kernel.org, linux-kselftest@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@surriel.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@suse.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Shuah Khan <shuah@kernel.org>
Subject: Re: [PATCH v1 3/3] mm/ksm: move disabling KSM from s390/gmap code
 to KSM code
Date:   Thu, 20 Apr 2023 14:41:00 -0700
In-reply-to: <20230418152849.505124-4-david@redhat.com>
Message-ID: <qvqwildqi62z.fsf@devbig1114.prn1.facebook.com>
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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)
>  {