[PATCH] arm64/mm: Optimize TLB flush in unmap_hotplug_[pmd|pud]_range()

[Anshuman Khandual]

flush_tlb_kernel_range() could flush down an entire block mapping just with
a single PAGE_SIZE stride. This capability was being used umapping PMD and
PUD based block mappings in unmap_hotplug_[pmd|pud]_range().

But later on the commit 48478b9f7913
("arm64/mm: Enable batched TLB flush in unmap_hotplug_range()") replaced
this PAGE_SIZE stride with [PMD|PUD]_SIZE strides, hence forcing multiple
PAGE_SIZE stride based TLB flushes on platforms where TLB range operation
is not supported. Revert back to the earlier TLB behaviour along with the
required comments that were dropped earlier.

Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will at kernel.org>
Cc: Ryan Roberts <ryan.roberts at arm.com>
Cc: David Hildenbrand <david at kernel.org>
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-kernel at vger.kernel.org
Reported-by: Ben Hutchings <ben at decadent.org.uk>
Closes: https://lore.kernel.org/all/b0d5836032ce3135bfc473f6bff791306d086925.camel@decadent.org.uk/
Fixes: 48478b9f7913 ("arm64/mm: Enable batched TLB flush in unmap_hotplug_range()")
Signed-off-by: Anshuman Khandual <anshuman.khandual at arm.com>
---
 arch/arm64/mm/mmu.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 8242f93f05e4..5ff0041f4a5f 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1509,7 +1509,11 @@ static void unmap_hotplug_pmd_range(pud_t *pudp, unsigned long addr,
 			if (free_mapped) {
 				/* CONT blocks are not supported in the vmemmap */
 				WARN_ON(pmd_cont(pmd));
-				flush_tlb_kernel_range(addr, addr + PMD_SIZE);
+				/*
+				 * One TLBI should be sufficient here as the PMD_SIZE
+				 * range is mapped with a single block entry.
+				 */
+				flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
 				free_hotplug_page_range(pmd_page(pmd),
 							PMD_SIZE, altmap);
 			}
@@ -1539,7 +1543,11 @@ static void unmap_hotplug_pud_range(p4d_t *p4dp, unsigned long addr,
 		if (pud_leaf(pud)) {
 			pud_clear(pudp);
 			if (free_mapped) {
-				flush_tlb_kernel_range(addr, addr + PUD_SIZE);
+				/*
+				 * One TLBI should be sufficient here as the PUD_SIZE
+				 * range is mapped with a single block entry.
+				 */
+				flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
 				free_hotplug_page_range(pud_page(pud),
 							PUD_SIZE, altmap);
 			}
-- 
2.30.2