[PATCH v2 6/6] migration/ram: Handle RAMBlocks with a RamDiscardManager on background snapshots

[David Hildenbrand <david@redhat.com>]

We already don't ever migrate memory that corresponds to discarded ranges
as managed by a RamDiscardManager responsible for the mapped memory region
of the RAMBlock.

virtio-mem uses this mechanism to logically unplug parts of a RAMBlock.
Right now, we still populate zeropages for the whole usable part of the
RAMBlock, which is undesired because:

1. Even populating the shared zeropage will result in memory getting
   consumed for page tables.
2. Memory backends without a shared zeropage (like hugetlbfs and shmem)
   will populate an actual, fresh page, resulting in an unintended
   memory consumption.

Discarded ("logically unplugged") parts have to remain discarded. As
these pages are never part of the migration stream, there is no need to
track modifications via userfaultfd WP reliably for these parts.

Further, any writes to these ranges by the VM are invalid and the
behavior is undefined.

Note that Linux only supports userfaultfd WP on private anonymous memory
for now, which usually results in the shared zeropage getting populated.
The issue will become more relevant once userfaultfd WP supports shmem
and hugetlb.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 migration/ram.c | 53 +++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 45 insertions(+), 8 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index d7505f5368..75de936bd2 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1612,6 +1612,28 @@ out:
     return ret;
 }
 
+static inline void populate_range(RAMBlock *block, hwaddr offset, hwaddr size)
+{
+    char *ptr = (char *) block->host;
+
+    for (; offset < size; offset += qemu_real_host_page_size) {
+        char tmp = *(ptr + offset);
+
+        /* Don't optimize the read out */
+        asm volatile("" : "+r" (tmp));
+    }
+}
+
+static inline int populate_section(MemoryRegionSection *section, void *opaque)
+{
+    const hwaddr size = int128_get64(section->size);
+    hwaddr offset = section->offset_within_region;
+    RAMBlock *block = section->mr->ram_block;
+
+    populate_range(block, offset, size);
+    return 0;
+}
+
 /*
  * ram_block_populate_pages: populate memory in the RAM block by reading
  *   an integer from the beginning of each page.
@@ -1621,16 +1643,31 @@ out:
  *
  * @block: RAM block to populate
  */
-static void ram_block_populate_pages(RAMBlock *block)
+static void ram_block_populate_pages(RAMBlock *rb)
 {
-    char *ptr = (char *) block->host;
-
-    for (ram_addr_t offset = 0; offset < block->used_length;
-            offset += qemu_real_host_page_size) {
-        char tmp = *(ptr + offset);
+    /*
+     * Skip populating all pages that fall into a discarded range as managed by
+     * a RamDiscardManager responsible for the mapped memory region of the
+     * RAMBlock. Such discarded ("logically unplugged") parts of a RAMBlock
+     * must not get populated automatically. We don't have to track
+     * modifications via userfaultfd WP reliably, because these pages will
+     * not be part of the migration stream either way -- see
+     * ramblock_dirty_bitmap_exclude_discarded_pages().
+     *
+     * Note: The result is only stable while migration (precopy/postcopy).
+     */
+    if (rb->mr && memory_region_has_ram_discard_manager(rb->mr)) {
+        RamDiscardManager *rdm = memory_region_get_ram_discard_manager(rb->mr);
+        MemoryRegionSection section = {
+            .mr = rb->mr,
+            .offset_within_region = 0,
+            .size = rb->mr->size,
+        };
 
-        /* Don't optimize the read out */
-        asm volatile("" : "+r" (tmp));
+        ram_discard_manager_replay_populated(rdm, &section,
+                                             populate_section, NULL);
+    } else {
+        populate_range(rb, 0, qemu_ram_get_used_length(rb));
     }
 }
 
-- 
2.31.1