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: <14d89518-0c11-7bfb-0c72-329a834ba1a1@redhat.com>
Date:   Fri, 21 Apr 2023 18:52:14 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Stefan Roesch <shr@...kernel.io>
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

[...]

>> 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,

David / dhildenb