[PATCH v5 9/9] 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.

Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 migration/ram.c | 38 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 36 insertions(+), 2 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index c212081f85..dbbb1e6712 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1656,6 +1656,17 @@ static inline void populate_read_range(RAMBlock *block, ram_addr_t offset,
     }
 }
 
+static inline int populate_read_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_read_range(block, offset, size);
+    return 0;
+}
+
 /*
  * ram_block_populate_read: preallocate page tables and populate pages in the
  *   RAM block by reading a byte of each page.
@@ -1665,9 +1676,32 @@ static inline void populate_read_range(RAMBlock *block, ram_addr_t offset,
  *
  * @block: RAM block to populate
  */
-static void ram_block_populate_read(RAMBlock *block)
+static void ram_block_populate_read(RAMBlock *rb)
 {
-    populate_read_range(block, 0, block->used_length);
+    /*
+     * 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 migrating (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,
+        };
+
+        ram_discard_manager_replay_populated(rdm, &section,
+                                             populate_read_section, NULL);
+    } else {
+        populate_read_range(rb, 0, rb->used_length);
+    }
 }
 
 /*
-- 
2.31.1