Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb MADV_DONTNEED processing

On Sun, Oct 30, 2022 at 06:44:10PM -0700, Mike Kravetz wrote:

On 10/30/22 11:52, Nadav Amit wrote:

...

On Oct 30, 2022, at 11:43 AM, Peter Xu peterx@redhat.com wrote:

...

The loop comes from 7e027b14d53e ("vm: simplify unmap_vmas() calling convention", 2012-05-06), where zap_page_range() was used to replace a call to unmap_vmas() because the patch wanted to eliminate the zap details pointer for unmap_vmas(), which makes sense.

I didn't check the old code, but from what I can tell (and also as Mike pointed out) I don't think zap_page_range() in the lastest code base is ever used on multi-vma at all. Otherwise the mmu notifier is already broken - see mmu_notifier_range_init() where the vma pointer is also part of the notification.

Perhaps we should just remove the loop?

There is already zap_page_range_single() that does exactly that. Just need to export it.

I was thinking that zap_page_range() should perform a notification call for each vma within the loop. Something like this?

I'm boldly guessing what Nadav suggested was using zap_page_range_single() and export it for MADV_DONTNEED. Hopefully that's also the easiest for stable?

For the long term, I really think we should just get rid of the loop..

@@ -1704,15 +1704,21 @@ void zap_page_range(struct vm_area_struct *vma, unsigned long start, MA_STATE(mas, mt, vma->vm_end, vma->vm_end); lru_add_drain();

• mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,

• 		start, start + size);
  

  tlb_gather_mmu(&tlb, vma->vm_mm); update_hiwater_rss(vma->vm_mm);
• mmu_notifier_invalidate_range_start(&range); do {

• unmap_single_vma(&tlb, vma, start, range.end, NULL);
  

• mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma,
  

• 		vma->vm_mm,
  

• 		max(start, vma->vm_start),
  

• 		min(start + size, vma->vm_end));
  

• if (is_vm_hugetlb_page(vma))
  

• 	adjust_range_if_pmd_sharing_possible(vma,
  

• 		&range.start,
  

• 		&range.end);
  

• mmu_notifier_invalidate_range_start(&range);
  

• unmap_single_vma(&tlb, vma, start, start + size, NULL);
  

• mmu_notifier_invalidate_range_end(&range);
  

  } while ((vma = mas_find(&mas, end - 1)) != NULL);

• mmu_notifier_invalidate_range_end(&range); tlb_finish_mmu(&tlb);

}

One thing to keep in mind is that this patch is a fix that must be backported to stable. Therefore, I do not think we want to add too many changes out of the direct scope of the fix.

We can always change things like this in follow up patches.

Mike Kravetz