[PATCH 12/16] mm/hmm: optimize migrate_vma_setup() for holes

[Ralph Campbell]

When migrating system memory to device private memory, if the source
address range is a valid VMA range and there is no memory or a zero page,
the source PFN array is marked as valid but with no PFN. This lets the
device driver allocate private memory and clear it, then insert the new
device private struct page into the CPU's page tables when
migrate_vma_pages() is called. migrate_vma_pages() only inserts the
new page if the VMA is an anonymous range. There is no point in telling
the device driver to allocate device private memory and then throwing
it away. Instead, mark the source PFN array entries as not migrating to
avoid this overhead.

Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
---
 mm/migrate.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 24535281cea3..87c52e0ee580 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2178,9 +2178,13 @@ static int migrate_vma_collect_hole(unsigned long start,
 {
 	struct migrate_vma *migrate = walk->private;
 	unsigned long addr;
+	unsigned long flags;
+
+	/* Only allow populating anonymous memory */
+	flags = vma_is_anonymous(walk->vma) ? MIGRATE_PFN_MIGRATE : 0;
 
 	for (addr = start; addr < end; addr += PAGE_SIZE) {
-		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
+		migrate->src[migrate->npages] = flags;
 		migrate->dst[migrate->npages] = 0;
 		migrate->npages++;
 		migrate->cpages++;
@@ -2748,7 +2752,7 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
 	pte_t *ptep;
 
 	/* Only allow populating anonymous memory */
-	if (!vma_is_anonymous(vma))
+	if (WARN_ON_ONCE(!vma_is_anonymous(vma)))
 		goto abort;
 
 	pgdp = pgd_offset(mm, addr);
-- 
2.20.1