overcommit: introduce mem-lock=on-fault

Locking the memory without MCL_ONFAULT instantly prefaults any mmaped
anonymous memory with a write-fault, which introduces a lot of extra
overhead in terms of memory usage when all you want to do is to prevent
kcompactd from migrating and compacting QEMU pages. Add an option to
only lock pages lazily as they're faulted by the process by using
MCL_ONFAULT if asked.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Link: https://lore.kernel.org/r/20250212143920.1269754-5-d-tatianin@yandex-team.ru
Signed-off-by: Peter Xu <peterx@redhat.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/system/globals.c b/system/globals.c
index adeff38348..316623bd20 100644
--- a/system/globals.c
+++ b/system/globals.c
@@ -33,7 +33,12 @@
 
 bool should_mlock(MlockState state)
 {
-    return state == MLOCK_ON;
+    return state == MLOCK_ON || state == MLOCK_ON_FAULT;
+}
+
+bool is_mlock_on_fault(MlockState state)
+{
+    return state == MLOCK_ON_FAULT;
 }
 
 enum vga_retrace_method vga_retrace_method = VGA_RETRACE_DUMB;