Re: [PATCH v5 1/4] os: add an ability to lock memory on_fault

[Philippe Mathieu-Daudé]

On 12/2/25 16:23, Peter Xu wrote:

On Wed, Feb 12, 2025 at 05:39:17PM +0300, Daniil Tatianin wrote:

This will be used in the following commits to make it possible to only
lock memory on fault instead of right away.

Signed-off-by: Daniil Tatianin <d-tatia...@yandex-team.ru>
---
  include/system/os-posix.h |  2 +-
  include/system/os-win32.h |  3 ++-
  meson.build               |  6 ++++++
  migration/postcopy-ram.c  |  2 +-
  os-posix.c                | 14 ++++++++++++--
  system/vl.c               |  2 +-
  6 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/os-posix.c b/os-posix.c
index 9cce55ff2f..17b144c0a2 100644
--- a/os-posix.c
+++ b/os-posix.c
@@ -327,18 +327,28 @@ void os_set_line_buffering(void)
      setvbuf(stdout, NULL, _IOLBF, 0);
  }

-int os_mlock(void)

+int os_mlock(bool on_fault)
  {
  #ifdef HAVE_MLOCKALL
      int ret = 0;
+    int flags = MCL_CURRENT | MCL_FUTURE;

- ret = mlockall(MCL_CURRENT | MCL_FUTURE);

+#ifdef HAVE_MLOCK_ONFAULT
+    if (on_fault) {
+        flags |= MCL_ONFAULT;
+    }
+#else
+    (void)on_fault;
+#endif

IIUC we should fail this when not supported.

Would you mind I squash this in?

diff --git a/os-posix.c b/os-posix.c
index 17b144c0a2..52925c23d3 100644
--- a/os-posix.c
+++ b/os-posix.c
@@ -333,13 +333,14 @@ int os_mlock(bool on_fault)
      int ret = 0;
      int flags = MCL_CURRENT | MCL_FUTURE;

-#ifdef HAVE_MLOCK_ONFAULT

      if (on_fault) {
+#ifdef HAVE_MLOCK_ONFAULT
          flags |= MCL_ONFAULT;
-    }
  #else
-    (void)on_fault;
+        error_report("mlockall: on_fault not supported");
+        return -EINVAL;

Good point!

  #endif
+    }

ret = mlockall(flags);

      if (ret < 0) {