[RFC PATCH v2 1/1] KVM: guest_memfd: Reorder undoing vmemmap optimization and stashing hugetlb folio metadata

[Ackerley Tng]

Without this patch, when HugeTLB folio metadata is stashed, the
vmemmap_optimized flag, stored in a HugeTLB folio's folio->private was
stashed as set.

The first splitting works, but on merging, when the folio metadata was
unstashed, vmemmap_optimized is unstashed as set, making the call to
hugetlb_vmemmap_optimize_folio() skip actually applying optimizations.

On a second split, hugetlb_vmemmap_restore_folio() attempts to reapply
optimizations when it was already applied, hence hitting the BUG().

Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
 mm/guestmem_hugetlb.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/mm/guestmem_hugetlb.c b/mm/guestmem_hugetlb.c
index 8727598cf18e..2c0192543676 100644
--- a/mm/guestmem_hugetlb.c
+++ b/mm/guestmem_hugetlb.c
@@ -200,16 +200,21 @@ static int guestmem_hugetlb_split_folio(struct folio *folio)
 		return 0;
 
 	orig_nr_pages = folio_nr_pages(folio);
-	ret = guestmem_hugetlb_stash_metadata(folio);
+
+	/*
+	 * hugetlb_vmemmap_restore_folio() has to be called ahead of the rest
+	 * because it checks page type. This doesn't actually split the folio,
+	 * so the first few struct pages are still intact.
+	 */
+	ret = hugetlb_vmemmap_restore_folio(folio_hstate(folio), folio);
 	if (ret)
 		return ret;
 
 	/*
-	 * hugetlb_vmemmap_restore_folio() has to be called ahead of the rest
-	 * because it checks and page type. This doesn't actually split the
-	 * folio, so the first few struct pages are still intact.
+	 * Stash metadata after vmemmap stuff so the outcome of the vmemmap
+	 * restoration is stashed.
 	 */
-	ret = hugetlb_vmemmap_restore_folio(folio_hstate(folio), folio);
+	ret = guestmem_hugetlb_stash_metadata(folio);
 	if (ret)
 		goto err;
 
@@ -254,8 +259,7 @@ static int guestmem_hugetlb_split_folio(struct folio *folio)
 	return 0;
 
 err:
-	guestmem_hugetlb_unstash_free_metadata(folio);
-
+	hugetlb_vmemmap_optimize_folio(folio_hstate(folio), folio);
 	return ret;
 }
 
-- 
2.49.0.1101.gccaa498523-goog