[PATCH v2 02/12] mm: hugetlb_vmemmap: unify VMEMMAP_{SPLIT,REMAP}_NO_TLB_FLUSH

[Muchun Song <songmuchun@bytedance.com>]

VMEMMAP_SPLIT_NO_TLB_FLUSH and VMEMMAP_REMAP_NO_TLB_FLUSH do not
describe fully independent choices. A split-only walk can defer the
PMD-split flush to a later global flush, and a remap-only walk can
defer the final remap flush. However, when a PMD split is followed
by PTE remapping, the split flush must happen before the PTEs are
changed, because stale large-page translations must not overlap with
new small-page translations that map different physical pages.

Replace the two flags with a single VMEMMAP_DEFER_TLB_FLUSH flag. The
new name reflects that only safely batchable flushes are deferred. Keep
split-only walks able to defer the flush, but force the split flush
whenever a remap_pte callback is present. Callers that batch HVO work
can still defer the final remap flush and issue their own global flush.

Current callers keep the same flushing behavior.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Qi Zheng <qi.zheng@linux.dev>
---
v2:
- Rename the unified flag to VMEMMAP_DEFER_TLB_FLUSH and update its
  comment (suggested by Qi Zheng)
- Update the commit message for the new flag name
- Collect Reviewed-by from Lance Yang and Qi Zheng
---
 mm/hugetlb_vmemmap.c | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 4b73d6ec8dce..63e899cfaac3 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -42,10 +42,8 @@ struct vmemmap_remap_walk {
 	struct list_head	*vmemmap_pages;
 
 
-/* Skip the TLB flush when we split the PMD */
-#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)
+/* Defer safely batchable TLB flushes to the caller. */
+#define VMEMMAP_DEFER_TLB_FLUSH	BIT(0)
 	unsigned long		flags;
 };
 
@@ -85,7 +83,12 @@ static int vmemmap_split_pmd(pmd_t *pmd, struct page *head, unsigned long start,
 		/* Make pte visible before pmd. See comment in pmd_install(). */
 		smp_wmb();
 		pmd_populate_kernel(&init_mm, pmd, pgtable);
-		if (!(walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH))
+		/*
+		 * Modern MMUs allow overlapping TLB entries with different mapping
+		 * granularities only if they map the same physical address. So
+		 * remapping must not proceed without first flushing the TLB.
+		 */
+		if (walk->remap_pte || !(walk->flags & VMEMMAP_DEFER_TLB_FLUSH))
 			flush_tlb_kernel_range(start, start + PMD_SIZE);
 	} else {
 		pte_free_kernel(&init_mm, pgtable);
@@ -166,7 +169,7 @@ static int vmemmap_remap_range(unsigned long start, unsigned long end,
 	if (ret)
 		return ret;
 
-	if (walk->remap_pte && !(walk->flags & VMEMMAP_REMAP_NO_TLB_FLUSH))
+	if (walk->remap_pte && !(walk->flags & VMEMMAP_DEFER_TLB_FLUSH))
 		flush_tlb_kernel_range(start, end);
 
 	return 0;
@@ -274,7 +277,7 @@ static int vmemmap_remap_split(unsigned long start, unsigned long end)
 {
 	struct vmemmap_remap_walk walk = {
 		.remap_pte	= NULL,
-		.flags		= VMEMMAP_SPLIT_NO_TLB_FLUSH,
+		.flags		= VMEMMAP_DEFER_TLB_FLUSH,
 	};
 
 	return vmemmap_remap_range(start, end, &walk);
@@ -458,7 +461,7 @@ 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;
+	unsigned long flags = VMEMMAP_DEFER_TLB_FLUSH;
 
 	list_for_each_entry_safe(folio, t_folio, folio_list, lru) {
 		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
@@ -516,7 +519,7 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 
 	/*
 	 * Very Subtle
-	 * If VMEMMAP_REMAP_NO_TLB_FLUSH is set, TLB flushing is not performed
+	 * If VMEMMAP_DEFER_TLB_FLUSH is set, TLB flushing is not performed
 	 * immediately after remapping.  As a result, subsequent accesses
 	 * and modifications to struct pages associated with the hugetlb
 	 * page could be to the OLD struct pages.  Set the vmemmap optimized
@@ -595,7 +598,7 @@ void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_l
 	struct folio *folio;
 	unsigned long nr_to_optimize = 0;
 	LIST_HEAD(vmemmap_pages);
-	unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH;
+	unsigned long flags = VMEMMAP_DEFER_TLB_FLUSH;
 
 	if (list_empty(folio_list))
 		return;
-- 
2.54.0