[PATCH RFC 3/3] mm/khugepaged: skip redundant IPI in collapse_huge_page()

[Lance Yang <lance.yang@linux.dev>]

From: Lance Yang <lance.yang@linux.dev>

Similar to the hugetlb PMD unsharing optimization, skip the second IPI
in collapse_huge_page() when the TLB flush already provides necessary
synchronization.

Before commit a37259732a7d ("x86/mm: Make MMU_GATHER_RCU_TABLE_FREE
unconditional"), bare metal x86 didn't enable MMU_GATHER_RCU_TABLE_FREE.
In that configuration, tlb_remove_table_sync_one() was a NOP. GUP-fast
synchronization relied on IRQ disabling, which blocks TLB flush IPIs.

When Rik made MMU_GATHER_RCU_TABLE_FREE unconditional to support AMD's
INVLPGB, all x86 systems started sending the second IPI. However, on
native x86 this is redundant:

  - pmdp_collapse_flush() calls flush_tlb_range(), sending IPIs to all
    CPUs to invalidate TLB entries

  - GUP-fast runs with IRQs disabled, so when the flush IPI completes,
    any concurrent GUP-fast must have finished

  - tlb_remove_table_sync_one() provides no additional synchronization

On x86, skip the second IPI when running native (without paravirt) and
without INVLPGB. For paravirt with non-native flush_tlb_multi and for
INVLPGB, conservatively keep both IPIs.

Use tlb_table_flush_implies_ipi_broadcast(), consistent with the hugetlb
optimization.

Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
 mm/khugepaged.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 97d1b2824386..06ea793a8190 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1178,7 +1178,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	_pmd = pmdp_collapse_flush(vma, address, pmd);
 	spin_unlock(pmd_ptl);
 	mmu_notifier_invalidate_range_end(&range);
-	tlb_remove_table_sync_one();
+	/*
+	 * Skip the second IPI if the TLB flush above already synchronized
+	 * with concurrent GUP-fast via broadcast IPIs.
+	 */
+	if (!tlb_table_flush_implies_ipi_broadcast())
+		tlb_remove_table_sync_one();
 
 	pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
 	if (pte) {
-- 
2.49.0