[Qemu-devel] [PATCHv3 7/9] migration: do not sent zero pages in bulk stage

[Peter Lieven <pl@kamp.de>]

during bulk stage of ram migration if a page is a
zero page do not send it at all.
the memory at the destination reads as zero anyway.

even if there is an madvise with QEMU_MADV_DONTNEED
at the target upon receipt of a zero page I have observed
that the target starts swapping if the memory is overcommitted.
it seems that the pages are dropped asynchronously.

Signed-off-by: Peter Lieven <pl@kamp.de>
---
 arch_init.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch_init.c b/arch_init.c
index 2b59454..c2cb40a 100644
--- a/arch_init.c
+++ b/arch_init.c
@@ -438,10 +438,12 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
             bytes_sent = -1;
             if (is_zero_page(p)) {
                 acct_info.dup_pages++;
-                bytes_sent = save_block_hdr(f, block, offset, cont,
-                                            RAM_SAVE_FLAG_COMPRESS);
-                qemu_put_byte(f, *p);
-                bytes_sent += 1;
+                if (!ram_bulk_stage) {
+                    bytes_sent = save_block_hdr(f, block, offset, cont,
+                                                RAM_SAVE_FLAG_COMPRESS);
+                    qemu_put_byte(f, 0);
+                }
+                bytes_sent++;
             } else if (migrate_use_xbzrle()) {
                 current_addr = block->offset + offset;
                 bytes_sent = save_xbzrle_page(f, p, current_addr, block,
-- 
1.7.9.5