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

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Message-ID: <qvqw1qkdt7iw.fsf@devbig1114.prn1.facebook.com>
Date:   Fri, 21 Apr 2023 11:27:23 -0700
From:   Stefan Roesch <shr@...kernel.io>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-s390@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...riel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v1 3/3] mm/ksm: move disabling KSM from s390/gmap code
 to KSM code


David Hildenbrand <david@...hat.com> writes:

> [...]
>
>>> 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;
>>>
>>
>
> Hi Stefan,
>
>> This clears the def_flags struct member, however, in ksm_disable() we
>> clear the __flags struct member. Is this a problem?
>
> The patch description contains a comment regarding def_flags: "The existing
> "mm->def_flags &= ~VM_MERGEABLE;" was essentially a NOP and can be dropped,
> because def_flags should never include VM_MERGEABLE."
>
> We keep clearing the MADV_UNMERGEABLE flag from MADV_UNMERGEABLE. In the old
> code, ksm_madvise() would have cleared it from local vm_flags and
> vm_flags_reset() would have modified vma->vm_flags. Now we clear it directly via
> vm_flags_clear(vma, VM_MERGEABLE);
>
>
> Long story short, the mm->def_flags code as wrong and most probably copied from
> thp_split_mm() where we do:
> 	mm->def_flags |= VM_NOHUGEPAGE;
> Which makes more sense.
>
> Thanks!

Thanks for the explanation.

Acked-by: Stefan Roesch <shr@...kernel.io>