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

[Peter Xu <peterx@redhat.com>]

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
> 

-- 
Peter Xu