mm/hugetlb_vmemmap: don't synchronize_rcu() without HVO

hugetlb_vmemmap_optimize_folio() and hugetlb_vmemmap_restore_folio() are
wrappers meant to be called regardless of whether HVO is enabled. 
Therefore, they should not call synchronize_rcu().  Otherwise, it
regresses use cases not enabling HVO.

So move synchronize_rcu() to __hugetlb_vmemmap_optimize_folio() and
__hugetlb_vmemmap_restore_folio(), and call it once for each batch of
folios when HVO is enabled.

Link: https://lkml.kernel.org/r/20240719042503.2752316-1-yuzhao@google.com
Fixes: bd225530 ("mm/hugetlb_vmemmap: fix race with speculative PFN walkers")
Signed-off-by: Yu Zhao <yuzhao@google.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202407091001.1250ad4a-oliver.sang@intel.comReported-by: Janosch Frank <frankja@linux.ibm.com>
Tested-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Acked-by: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb_vmemmap.c

@@ -43,6 +43,8 @@ struct vmemmap_remap_walk {
 #define VMEMMAP_SPLIT_NO_TLB_FLUSH	BIT(0)
 /* Skip the TLB flush when we remap the PTE */
 #define VMEMMAP_REMAP_NO_TLB_FLUSH	BIT(1)
+/* synchronize_rcu() to avoid writes from page_ref_add_unless() */
+#define VMEMMAP_SYNCHRONIZE_RCU		BIT(2)
 	unsigned long		flags;
 };
 
@@ -457,6 +459,9 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
 	if (!folio_test_hugetlb_vmemmap_optimized(folio))
 		return 0;
 
+	if (flags & VMEMMAP_SYNCHRONIZE_RCU)
+		synchronize_rcu();
+
 	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
 	vmemmap_reuse	= vmemmap_start;
 	vmemmap_start	+= HUGETLB_VMEMMAP_RESERVE_SIZE;
@@ -489,10 +494,7 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
  */
 int hugetlb_vmemmap_restore_folio(const struct hstate *h, struct folio *folio)
 {
-	/* avoid writes from page_ref_add_unless() while unfolding vmemmap */
-	synchronize_rcu();
-
-	return __hugetlb_vmemmap_restore_folio(h, folio, 0);
+	return __hugetlb_vmemmap_restore_folio(h, folio, VMEMMAP_SYNCHRONIZE_RCU);
 }
 
 /**
@@ -515,14 +517,14 @@ long hugetlb_vmemmap_restore_folios(const struct hstate *h,
 	struct folio *folio, *t_folio;
 	long restored = 0;
 	long ret = 0;
-
-	/* avoid writes from page_ref_add_unless() while unfolding vmemmap */
-	synchronize_rcu();
+	unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;
 
 	list_for_each_entry_safe(folio, t_folio, folio_list, lru) {
 		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
-			ret = __hugetlb_vmemmap_restore_folio(h, folio,
-							      VMEMMAP_REMAP_NO_TLB_FLUSH);
+			ret = __hugetlb_vmemmap_restore_folio(h, folio, flags);
+			/* only need to synchronize_rcu() once for each batch */
+			flags &= ~VMEMMAP_SYNCHRONIZE_RCU;
+
 			if (ret)
 				break;
 			restored++;
@@ -570,6 +572,9 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 		return ret;
 
 	static_branch_inc(&hugetlb_optimize_vmemmap_key);
+
+	if (flags & VMEMMAP_SYNCHRONIZE_RCU)
+		synchronize_rcu();
 	/*
 	 * Very Subtle
 	 * If VMEMMAP_REMAP_NO_TLB_FLUSH is set, TLB flushing is not performed
@@ -617,10 +622,7 @@ void hugetlb_vmemmap_optimize_folio(const struct hstate *h, struct folio *folio)
 {
 	LIST_HEAD(vmemmap_pages);
 
-	/* avoid writes from page_ref_add_unless() while folding vmemmap */
-	synchronize_rcu();
-
-	__hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, 0);
+	__hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, VMEMMAP_SYNCHRONIZE_RCU);
 	free_vmemmap_page_list(&vmemmap_pages);
 }
 
@@ -647,6 +649,7 @@ void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_l
 {
 	struct folio *folio;
 	LIST_HEAD(vmemmap_pages);
+	unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;
 
 	list_for_each_entry(folio, folio_list, lru) {
 		int ret = hugetlb_vmemmap_split_folio(h, folio);
@@ -663,14 +666,12 @@ void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_l
 
 	flush_tlb_all();
 
-	/* avoid writes from page_ref_add_unless() while folding vmemmap */
-	synchronize_rcu();
-
 	list_for_each_entry(folio, folio_list, lru) {
 		int ret;
 
-		ret = __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages,
-						       VMEMMAP_REMAP_NO_TLB_FLUSH);
+		ret = __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, flags);
+		/* only need to synchronize_rcu() once for each batch */
+		flags &= ~VMEMMAP_SYNCHRONIZE_RCU;
 
 		/*
 		 * Pages to be freed may have been accumulated.  If we
@@ -684,8 +685,7 @@ void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_l
 			flush_tlb_all();
 			free_vmemmap_page_list(&vmemmap_pages);
 			INIT_LIST_HEAD(&vmemmap_pages);
-			__hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages,
-							 VMEMMAP_REMAP_NO_TLB_FLUSH);
+			__hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, flags);
 		}
 	}