migration/ram: Rely on used_length for uffd_change_protection()

ram_mig_ram_block_resized() will abort migration (including background
snapshots) when resizing a RAMBlock. ram_block_populate_read() will only
populate RAM up to used_length, so at least for anonymous memory
protecting everything between used_length and max_length won't
actually be protected and is just a NOP.

So let's only protect everything up to used_length.

Note: it still makes sense to register uffd-wp for max_length, such
that RAM_UF_WRITEPROTECT is independent of a changing used_length.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

diff --git a/migration/ram.c b/migration/ram.c
index efaae07dd8040ea0de90d2f6f0adbd5d9e89fedf..a6956c9e7d05e56dbe1cffad4c896e2a9519cd81 100644 (file)
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1901,7 +1901,7 @@ int ram_write_tracking_start(void)
 
         /* Apply UFFD write protection to the block memory range */
         if (uffd_change_protection(rs->uffdio_fd, block->host,
-                block->max_length, true, false)) {
+                                   block->used_length, true, false)) {
             goto fail;
         }