[Qemu-devel] [PATCH v8 51/54] Postcopy: Mark nohugepage before discard

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

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

Prior to servicing userfault requests we must ensure we've not got
huge pages in the area that might include non-transferred memory,
since a hugepage could incorrectly mark the whole huge page as present.

We mark the area as non-huge page (nhp) just before we perform
discards; the discard code now tells us to discard any areas
that haven't been sent (as well as any that are redirtied);
any already formed transparent-huge-pages get fragmented
by this discard process if they cotnain any discards.

Transparent huge pages that have been entirely transferred
and don't contain any discards are not broken by this mechanism;
they stay as huge pages.

By starting postcopy after a full precopy pass, many of the pages
then stay as huge pages; this is important for maintaining performance
after the end of the migration.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 include/migration/postcopy-ram.h |  6 ++++++
 migration/postcopy-ram.c         | 46 +++++++++++++++++++++++++++++++++++++---
 migration/savevm.c               |  9 +++++++-
 trace-events                     |  1 +
 4 files changed, 58 insertions(+), 4 deletions(-)

diff --git a/include/migration/postcopy-ram.h b/include/migration/postcopy-ram.h
index 50c1ce5..60ac5b1 100644
--- a/include/migration/postcopy-ram.h
+++ b/include/migration/postcopy-ram.h
@@ -41,6 +41,12 @@ int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis);
 int postcopy_ram_discard_range(MigrationIncomingState *mis, uint8_t *start,
                                size_t length);
 
+/*
+ * Userfault requires us to mark RAM as NOHUGEPAGE prior to discard
+ * however leaving it until after precopy means that most of the precopy
+ * data is still THPd
+ */
+int postcopy_ram_prepare_discard(MigrationIncomingState *mis);
 
 /*
  * Called at the start of each RAMBlock by the bitmap code.
diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index 0b021ca..10e5a5b 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -226,12 +226,10 @@ static int cleanup_range(const char *block_name, void *host_addr,
      * We turned off hugepage for the precopy stage with postcopy enabled
      * we can turn it back on now.
      */
-#ifdef MADV_HUGEPAGE
-    if (madvise(host_addr, length, MADV_HUGEPAGE)) {
+    if (qemu_madvise(host_addr, length, QEMU_MADV_HUGEPAGE)) {
         error_report("%s HUGEPAGE: %s", __func__, strerror(errno));
         return -1;
     }
-#endif
 
     /*
      * We can also turn off userfault now since we should have all the
@@ -308,6 +306,43 @@ int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis)
 }
 
 /*
+ * Disable huge pages on an area
+ */
+static int nhp_range(const char *block_name, void *host_addr,
+                    ram_addr_t offset, ram_addr_t length, void *opaque)
+{
+    trace_postcopy_nhp_range(block_name, host_addr, offset, length);
+
+    /*
+     * Before we do discards we need to ensure those discards really
+     * do delete areas of the page, even if THP thinks a hugepage would
+     * be a good idea, so force hugepages off.
+     */
+    if (qemu_madvise(host_addr, length, QEMU_MADV_NOHUGEPAGE)) {
+        error_report("%s: NOHUGEPAGE: %s", __func__, strerror(errno));
+        return -1;
+    }
+
+    return 0;
+}
+
+/*
+ * Userfault requires us to mark RAM as NOHUGEPAGE prior to discard
+ * however leaving it until after precopy means that most of the precopy
+ * data is still THPd
+ */
+int postcopy_ram_prepare_discard(MigrationIncomingState *mis)
+{
+    if (qemu_ram_foreach_block(nhp_range, mis)) {
+        return -1;
+    }
+
+    postcopy_state_set(POSTCOPY_INCOMING_DISCARD);
+
+    return 0;
+}
+
+/*
  * Mark the given area of RAM as requiring notification to unwritten areas
  * Used as a  callback on qemu_ram_foreach_block.
  *   host_addr: Base of area to mark
@@ -581,6 +616,11 @@ int postcopy_ram_discard_range(MigrationIncomingState *mis, uint8_t *start,
     assert(0);
 }
 
+int postcopy_ram_prepare_discard(MigrationIncomingState *mis)
+{
+    assert(0);
+}
+
 int postcopy_ram_enable_notify(MigrationIncomingState *mis)
 {
     assert(0);
diff --git a/migration/savevm.c b/migration/savevm.c
index 7c5b9d1..894e085 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1314,7 +1314,7 @@ static int loadvm_postcopy_ram_handle_discard(MigrationIncomingState *mis,
     switch (ps) {
     case POSTCOPY_INCOMING_ADVISE:
         /* 1st discard */
-        tmp = 0; /* TODO: later patch postcopy_ram_prepare_discard(mis); */
+        tmp = postcopy_ram_prepare_discard(mis);
         if (tmp) {
             return tmp;
         }
@@ -1446,6 +1446,13 @@ static int loadvm_postcopy_handle_listen(MigrationIncomingState *mis)
         error_report("CMD_POSTCOPY_LISTEN in wrong postcopy state (%d)", ps);
         return -1;
     }
+    if (ps == POSTCOPY_INCOMING_ADVISE) {
+        /*
+         * A rare case, we entered listen without having to do any discards,
+         * so do the setup that's normally done at the time of the 1st discard.
+         */
+        postcopy_ram_prepare_discard(mis);
+    }
 
     /*
      * Sensitise RAM - can now generate requests for blocks that don't exist
diff --git a/trace-events b/trace-events
index 42a7577..81f9ca7 100644
--- a/trace-events
+++ b/trace-events
@@ -1542,6 +1542,7 @@ postcopy_discard_send_finish(const char *ramblock, int nwords, int ncmds) "%s ma
 postcopy_ram_discard_range(void *start, size_t length) "%p,+%zx"
 postcopy_cleanup_range(const char *ramblock, void *host_addr, size_t offset, size_t length) "%s: %p offset=%zx length=%zx"
 postcopy_init_range(const char *ramblock, void *host_addr, size_t offset, size_t length) "%s: %p offset=%zx length=%zx"
+postcopy_nhp_range(const char *ramblock, void *host_addr, size_t offset, size_t length) "%s: %p offset=%zx length=%zx"
 postcopy_place_page(void *host_addr) "host=%p"
 postcopy_place_page_zero(void *host_addr) "host=%p"
 postcopy_ram_enable_notify(void) ""
-- 
2.5.0