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

[Naoya Horiguchi]

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?

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