Re: [PATCH v2 01/11] hugetlb: set hugetlb page flag before optimizing vmemmap

[Mike Kravetz]

On 10/13/23 21:58, Naoya Horiguchi wrote:
> On Tue, Sep 05, 2023 at 02:44:00PM -0700, Mike Kravetz wrote:
> > Currently, vmemmap optimization of hugetlb pages is performed before the
> > hugetlb flag (previously hugetlb destructor) is set identifying it as a
> > hugetlb folio.  This means there is a window of time where an ordinary
> > folio does not have all associated vmemmap present.  The core mm only
> > expects vmemmap to be potentially optimized for hugetlb  and device dax.
> > This can cause problems in code such as memory error handling that may
> > want to write to tail struct pages.
> > 
> > There is only one call to perform hugetlb vmemmap optimization today.
> > To fix this issue, simply set the hugetlb flag before that call.
> > 
> > There was a similar issue in the free hugetlb path that was previously
> > addressed.  The two routines that optimize or restore hugetlb vmemmap
> > should only be passed hugetlb folios/pages.  To catch any callers not
> > following this rule, add VM_WARN_ON calls to the routines.  In the
> > hugetlb free code paths, some calls could be made to restore vmemmap
> > after clearing the hugetlb flag.  This was 'safe' as in these cases
> > vmemmap was already present and the call was a NOOP.  However, for
> > consistency these calls where eliminated so that we can add the
> > VM_WARN_ON checks.
> > 
> > Fixes: f41f2ed43ca5 ("mm: hugetlb: free the vmemmap pages associated with each HugeTLB page")
> > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> 
> I saw that VM_WARN_ON_ONCE() in hugetlb_vmemmap_restore is triggered when
> memory_failure() is called on a free hugetlb page with vmemmap optimization
> disabled (the warning is not triggered if vmemmap optimization is enabled).
> I think that we need check folio_test_hugetlb() before dissolve_free_huge_page()
> calls hugetlb_vmemmap_restore_folio().
> 
> Could you consider adding some diff like below?

Thanks!  That case was indeed overlooked.

Andrew, this patch is currently in mm-stable.  How would you like to update?
- A new version of the patch
- An patch to the original patch
- Something else

-- 
Mike Kravetz

> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2312,15 +2312,16 @@ int dissolve_free_huge_page(struct page *page)
>  		 * Attempt to allocate vmemmmap here so that we can take
>  		 * appropriate action on failure.
>  		 */
> -		rc = hugetlb_vmemmap_restore_folio(h, folio);
> -		if (!rc) {
> -			update_and_free_hugetlb_folio(h, folio, false);
> -		} else {
> -			spin_lock_irq(&hugetlb_lock);
> -			add_hugetlb_folio(h, folio, false);
> -			h->max_huge_pages++;
> -			spin_unlock_irq(&hugetlb_lock);
> +		if (folio_test_hugetlb(folio)) {
> +			rc = hugetlb_vmemmap_restore_folio(h, folio);
> +			if (rc) {
> +				spin_lock_irq(&hugetlb_lock);
> +				add_hugetlb_folio(h, folio, false);
> +				h->max_huge_pages++;
> +				goto out;
> +			}
>  		}
> +		update_and_free_hugetlb_folio(h, folio, false);
>  
>  		return rc;
>  	}
> 
> 
> Thanks,
> Naoya Horiguchi