s390/vmemmap: avoid memset(PAGE_UNUSED) when adding consecutive sections

Let's avoid memset(PAGE_UNUSED) when adding consecutive sections,
whereby the vmemmap of a single section does not span full PMDs.

Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200722094558.9828-10-david@redhat.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/mm/vmem.c

@@ -74,7 +74,22 @@ static void vmem_pte_free(unsigned long *table)
 
 #define PAGE_UNUSED 0xFD
 
-static void vmemmap_use_sub_pmd(unsigned long start, unsigned long end)
+/*
+ * The unused vmemmap range, which was not yet memset(PAGE_UNUSED) ranges
+ * from unused_pmd_start to next PMD_SIZE boundary.
+ */
+static unsigned long unused_pmd_start;
+
+static void vmemmap_flush_unused_pmd(void)
+{
+	if (!unused_pmd_start)
+		return;
+	memset(__va(unused_pmd_start), PAGE_UNUSED,
+	       ALIGN(unused_pmd_start, PMD_SIZE) - unused_pmd_start);
+	unused_pmd_start = 0;
+}
+
+static void __vmemmap_use_sub_pmd(unsigned long start, unsigned long end)
 {
 	/*
 	 * As we expect to add in the same granularity as we remove, it's
@@ -85,18 +100,41 @@ static void vmemmap_use_sub_pmd(unsigned long start, unsigned long end)
 	memset(__va(start), 0, sizeof(struct page));
 }
 
+static void vmemmap_use_sub_pmd(unsigned long start, unsigned long end)
+{
+	/*
+	 * We only optimize if the new used range directly follows the
+	 * previously unused range (esp., when populating consecutive sections).
+	 */
+	if (unused_pmd_start == start) {
+		unused_pmd_start = end;
+		if (likely(IS_ALIGNED(unused_pmd_start, PMD_SIZE)))
+			unused_pmd_start = 0;
+		return;
+	}
+	vmemmap_flush_unused_pmd();
+	__vmemmap_use_sub_pmd(start, end);
+}
+
 static void vmemmap_use_new_sub_pmd(unsigned long start, unsigned long end)
 {
 	void *page = __va(ALIGN_DOWN(start, PMD_SIZE));
 
+	vmemmap_flush_unused_pmd();
+
 	/* Could be our memmap page is filled with PAGE_UNUSED already ... */
-	vmemmap_use_sub_pmd(start, end);
+	__vmemmap_use_sub_pmd(start, end);
 
 	/* Mark the unused parts of the new memmap page PAGE_UNUSED. */
 	if (!IS_ALIGNED(start, PMD_SIZE))
 		memset(page, PAGE_UNUSED, start - __pa(page));
+	/*
+	 * We want to avoid memset(PAGE_UNUSED) when populating the vmemmap of
+	 * consecutive sections. Remember for the last added PMD the last
+	 * unused range in the populated PMD.
+	 */
 	if (!IS_ALIGNED(end, PMD_SIZE))
-		memset(__va(end), PAGE_UNUSED, __pa(page) + PMD_SIZE - end);
+		unused_pmd_start = end;
 }
 
 /* Returns true if the PMD is completely unused and can be freed. */
@@ -104,6 +142,7 @@ static bool vmemmap_unuse_sub_pmd(unsigned long start, unsigned long end)
 {
 	void *page = __va(ALIGN_DOWN(start, PMD_SIZE));
 
+	vmemmap_flush_unused_pmd();
 	memset(__va(start), PAGE_UNUSED, end - start);
 	return !memchr_inv(page, PAGE_UNUSED, PMD_SIZE);
 }