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

[David Hildenbrand]

[...]

> > 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