[PATCH v5 8/9] migration/ram: Factor out populating pages readable in ram_block_populate_pages()

[David Hildenbrand <david@redhat.com>]

Let's factor out prefaulting/populating to make further changes easier to
review and add a comment what we are actually expecting to happen. While at
it, use the actual page size of the ramblock, which defaults to
qemu_real_host_page_size for anonymous memory. Further, rename
ram_block_populate_pages() to ram_block_populate_read() as well, to make
it clearer what we are doing.

In the future, we might want to use MADV_POPULATE_READ to speed up
population.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 migration/ram.c | 35 ++++++++++++++++++++++-------------
 1 file changed, 22 insertions(+), 13 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index b225ec7507..c212081f85 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1639,26 +1639,35 @@ out:
     return ret;
 }
 
+static inline void populate_read_range(RAMBlock *block, ram_addr_t offset,
+                                       ram_addr_t size)
+{
+    /*
+     * We read one byte of each page; this will preallocate page tables if
+     * required and populate the shared zeropage on MAP_PRIVATE anonymous memory
+     * where no page was populated yet. This might require adaption when
+     * supporting other mappings, like shmem.
+     */
+    for (; offset < size; offset += block->page_size) {
+        char tmp = *((char *)block->host + offset);
+
+        /* Don't optimize the read out */
+        asm volatile("" : "+r" (tmp));
+    }
+}
+
 /*
- * ram_block_populate_pages: populate memory in the RAM block by reading
- *   an integer from the beginning of each page.
+ * ram_block_populate_read: preallocate page tables and populate pages in the
+ *   RAM block by reading a byte of each page.
  *
  * Since it's solely used for userfault_fd WP feature, here we just
  *   hardcode page size to qemu_real_host_page_size.
  *
  * @block: RAM block to populate
  */
-static void ram_block_populate_pages(RAMBlock *block)
+static void ram_block_populate_read(RAMBlock *block)
 {
-    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);
-
-        /* Don't optimize the read out */
-        asm volatile("" : "+r" (tmp));
-    }
+    populate_read_range(block, 0, block->used_length);
 }
 
 /*
@@ -1684,7 +1693,7 @@ void ram_write_tracking_prepare(void)
          * UFFDIO_WRITEPROTECT_MODE_WP mode setting would silently skip
          * pages with pte_none() entries in page table.
          */
-        ram_block_populate_pages(block);
+        ram_block_populate_read(block);
     }
 }
 
-- 
2.31.1