[PATCH 1/2] arm64/mm: Enable batched TLB flush in unmap_hotplug_range()

[Anshuman Khandual <anshuman.khandual@arm.com>]

During a memory hot remove operartion both linear and vmemmap mappings for
the memory range being removed, get unmapped via unmap_hotplug_range() but
mapped pages get freed only for vmemmap mapping. This is just a sequential
operation where each table entry gets cleared, followed by a leaf specific
TLB flush, and then followed by memory free operation when applicable.

This approach was simple and uniform both for vmemmap and linear mappings.
But linear mapping might contain CONT marked block memory where it becomes
necessary to first clear out all entire in the range before a TLB flush.
This is as per the architecture requirement. Hence batch all TLB flushes
during the table tear down walk and finally do it in unmap_hotplug_range().

Besides it is helps in improving the performance via TLBI range operation
along with reduced synchronization instructions. The time spent executing
unmap_hotplug_range() improved 97% measured over a 2GB memory hot removal
in KVM guest.

This scheme is not applicable during vmemmap mapping tear down where memory
needs to be freed and hence a TLB flush is required after clearing out page
table entry.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Closes: https://lore.kernel.org/all/aWZYXhrT6D2M-7-N@willie-the-truck/
Fixes: bbd6ec605c0f ("arm64/mm: Enable memory hot remove")
Cc: stable@vger.kernel.org
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 arch/arm64/mm/mmu.c | 81 +++++++++++++++++++++++++++++++++++++--------
 1 file changed, 67 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 8e1d80a7033e..8ec8a287aaa1 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1458,10 +1458,32 @@ static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
 
 		WARN_ON(!pte_present(pte));
 		__pte_clear(&init_mm, addr, ptep);
-		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
-		if (free_mapped)
+		if (free_mapped) {
+			/*
+			 * If page is part of an existing contiguous
+			 * memory block, individual TLB invalidation
+			 * here would not be appropriate. Instead it
+			 * will require clearing all entries for the
+			 * memory block and subsequently a TLB flush
+			 * for the entire range.
+			 */
+			WARN_ON(pte_cont(pte));
+
+			/*
+			 * TLB flush is essential for freeing memory.
+			 */
+			flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
 			free_hotplug_page_range(pte_page(pte),
 						PAGE_SIZE, altmap);
+		}
+
+		/*
+		 * TLB flush is batched in unmap_hotplug_range()
+		 * for the entire range, when memory need not be
+		 * freed. Besides linear mapping might have CONT
+		 * blocks where TLB flush needs to be done after
+		 * clearing all relevant entries.
+		 */
 	} while (addr += PAGE_SIZE, addr < end);
 }
 
@@ -1482,15 +1504,32 @@ static void unmap_hotplug_pmd_range(pud_t *pudp, unsigned long addr,
 		WARN_ON(!pmd_present(pmd));
 		if (pmd_sect(pmd)) {
 			pmd_clear(pmdp);
+			if (free_mapped) {
+				/*
+				 * If page is part of an existing contiguous
+				 * memory block, individual TLB invalidation
+				 * here would not be appropriate. Instead it
+				 * will require clearing all entries for the
+				 * memory block and subsequently a TLB flush
+				 * for the entire range.
+				 */
+				WARN_ON(pmd_cont(pmd));
+
+				/*
+				 * TLB flush is essential for freeing memory.
+				 */
+				flush_tlb_kernel_range(addr, addr + PMD_SIZE);
+				free_hotplug_page_range(pmd_page(pmd),
+							PMD_SIZE, altmap);
+			}
 
 			/*
-			 * One TLBI should be sufficient here as the PMD_SIZE
-			 * range is mapped with a single block entry.
+			 * TLB flush is batched in unmap_hotplug_range()
+			 * for the entire range, when memory need not be
+			 * freed. Besides linear mapping might have CONT
+			 * blocks where TLB flush needs to be done after
+			 * clearing all relevant entries.
 			 */
-			flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
-			if (free_mapped)
-				free_hotplug_page_range(pmd_page(pmd),
-							PMD_SIZE, altmap);
 			continue;
 		}
 		WARN_ON(!pmd_table(pmd));
@@ -1515,15 +1554,20 @@ static void unmap_hotplug_pud_range(p4d_t *p4dp, unsigned long addr,
 		WARN_ON(!pud_present(pud));
 		if (pud_sect(pud)) {
 			pud_clear(pudp);
+			if (free_mapped) {
+				/*
+				 * TLB flush is essential for freeing memory.
+				 */
+				flush_tlb_kernel_range(addr, addr + PUD_SIZE);
+				free_hotplug_page_range(pud_page(pud),
+							PUD_SIZE, altmap);
+			}
 
 			/*
-			 * One TLBI should be sufficient here as the PUD_SIZE
-			 * range is mapped with a single block entry.
+			 * TLB flush is batched in unmap_hotplug_range()
+			 * for the entire range, when memory need not be
+			 * freed.
 			 */
-			flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
-			if (free_mapped)
-				free_hotplug_page_range(pud_page(pud),
-							PUD_SIZE, altmap);
 			continue;
 		}
 		WARN_ON(!pud_table(pud));
@@ -1553,6 +1597,7 @@ static void unmap_hotplug_p4d_range(pgd_t *pgdp, unsigned long addr,
 static void unmap_hotplug_range(unsigned long addr, unsigned long end,
 				bool free_mapped, struct vmem_altmap *altmap)
 {
+	unsigned long start = addr;
 	unsigned long next;
 	pgd_t *pgdp, pgd;
 
@@ -1574,6 +1619,14 @@ static void unmap_hotplug_range(unsigned long addr, unsigned long end,
 		WARN_ON(!pgd_present(pgd));
 		unmap_hotplug_p4d_range(pgdp, addr, next, free_mapped, altmap);
 	} while (addr = next, addr < end);
+
+	/*
+	 * Batched TLB flush only for linear mapping which
+	 * might contain CONT blocks, and does not require
+	 * freeing up memory as well.
+	 */
+	if (!free_mapped)
+		flush_tlb_kernel_range(start, end);
 }
 
 static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
-- 
2.30.2