[PATCHv7 14/18] hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU

Kiryl Shutsemau (Meta)
2026-02-27T19:30:56Z

From: Kiryl Shutsemau <kas@kernel.org>

The VMEMMAP_SYNCHRONIZE_RCU flag triggered synchronize_rcu() calls to
prevent a race between HVO remapping and page_ref_add_unless(). The
race could occur when a speculative PFN walker tried to modify the
refcount on a struct page that was in the process of being remapped
to a fake head.

With fake heads eliminated, page_ref_add_unless() no longer needs RCU
protection.

Remove the flag and synchronize_rcu() calls.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
---
mm/hugetlb_vmemmap.c | 20 ++++----------------
1 file changed, 4 insertions(+), 16 deletions(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 92330f172eb7..fd1d5d5d12b4 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -47,8 +47,6 @@ 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;
};

@@ -409,9 +407,6 @@ 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_start = (unsigned long)&folio->page;
vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);

@@ -444,7 +439,7 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
*/
int hugetlb_vmemmap_restore_folio(const struct hstate *h, struct folio *folio)
{
- return __hugetlb_vmemmap_restore_folio(h, folio, VMEMMAP_SYNCHRONIZE_RCU);
+ return __hugetlb_vmemmap_restore_folio(h, folio, 0);
}

/**
@@ -467,14 +462,11 @@ long hugetlb_vmemmap_restore_folios(const struct hstate *h,
struct folio *folio, *t_folio;
long restored = 0;
long ret = 0;
- unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;
+ unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH;

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, flags);
- /* only need to synchronize_rcu() once for each batch */
- flags &= ~VMEMMAP_SYNCHRONIZE_RCU;
-
if (ret)
break;
restored++;
@@ -554,8 +546,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,

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
@@ -613,7 +603,7 @@ void hugetlb_vmemmap_optimize_folio(const struct hstate *h, struct folio *folio)
{
LIST_HEAD(vmemmap_pages);

- __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, VMEMMAP_SYNCHRONIZE_RCU);
+ __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, 0);
free_vmemmap_page_list(&vmemmap_pages);
}

@@ -641,7 +631,7 @@ static void __hugetlb_vmemmap_optimize_folios(struct hstate *h,
struct folio *folio;
int nr_to_optimize;
LIST_HEAD(vmemmap_pages);
- unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;
+ unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH;

nr_to_optimize = 0;
list_for_each_entry(folio, folio_list, lru) {
@@ -694,8 +684,6 @@ static void __hugetlb_vmemmap_optimize_folios(struct hstate *h,
int ret;

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
--
2.51.2