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

Stefan Roesch
2023-04-21T18:28:06Z

David Hildenbrand <david@redhat.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@devkernel.io>