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

David Hildenbrand
2023-04-21T17:08:06Z

[...]

>> 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