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

[Mike Kravetz]

On 10/30/22 11:52, Nadav Amit wrote:
> On Oct 30, 2022, at 11:43 AM, Peter Xu <peterx@xxxxxxxxxx> 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?

@@ -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