[Qemu-devel] [PATCH 08/15] postcopy: Use temporary for placing zero huge pages

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

The kernel can't do UFFDIO_ZEROPAGE for huge pages, so we have
to allocate a temporary (always zero) page and use UFFDIO_COPYPAGE
on it.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 include/migration/migration.h |  1 +
 migration/postcopy-ram.c      | 23 +++++++++++++++++++++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/migration/migration.h b/include/migration/migration.h
index 7b311dd..a1633de 100644
--- a/include/migration/migration.h
+++ b/include/migration/migration.h
@@ -105,6 +105,7 @@ struct MigrationIncomingState {
     QEMUFile *to_src_file;
     QemuMutex rp_mutex;    /* We send replies from multiple threads */
     void     *postcopy_tmp_page;
+    void     *postcopy_tmp_zero_page;
 
     QEMUBH *bh;
 
diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index a8b7fed..4c736d2 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -324,6 +324,10 @@ int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis)
         munmap(mis->postcopy_tmp_page, mis->largest_page_size);
         mis->postcopy_tmp_page = NULL;
     }
+    if (mis->postcopy_tmp_zero_page) {
+        munmap(mis->postcopy_tmp_zero_page, mis->largest_page_size);
+        mis->postcopy_tmp_zero_page = NULL;
+    }
     trace_postcopy_ram_incoming_cleanup_exit();
     return 0;
 }
@@ -593,8 +597,23 @@ int postcopy_place_page_zero(MigrationIncomingState *mis, void *host,
             return -e;
         }
     } else {
-        /* TODO: The kernel can't use UFFDIO_ZEROPAGE for hugepages */
-        assert(0);
+        /* The kernel can't use UFFDIO_ZEROPAGE for hugepages */
+        if (!mis->postcopy_tmp_zero_page) {
+            mis->postcopy_tmp_zero_page = mmap(NULL, mis->largest_page_size,
+                                               PROT_READ | PROT_WRITE,
+                                               MAP_PRIVATE | MAP_ANONYMOUS,
+                                               -1, 0);
+            if (mis->postcopy_tmp_zero_page == MAP_FAILED) {
+                int e = errno;
+                mis->postcopy_tmp_zero_page = NULL;
+                error_report("%s: %s mapping large zero page",
+                             __func__, strerror(e));
+                return -e;
+            }
+            memset(mis->postcopy_tmp_zero_page, '\0', mis->largest_page_size);
+        }
+        return postcopy_place_page(mis, host, mis->postcopy_tmp_zero_page,
+                                   pagesize);
     }
 
     return 0;
-- 
2.9.3