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

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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




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