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

[Daniil Tatianin]

On 2/12/25 6:23 PM, 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/include/system/os-posix.h b/include/system/os-posix.h
index b881ac6c6f..ce5b3bccf8 100644
--- a/include/system/os-posix.h
+++ b/include/system/os-posix.h
@@ -53,7 +53,7 @@ bool os_set_runas(const char *user_id);
  void os_set_chroot(const char *path);
  void os_setup_limits(void);
  void os_setup_post(void);
-int os_mlock(void);
+int os_mlock(bool on_fault);

/**

   * qemu_alloc_stack:
diff --git a/include/system/os-win32.h b/include/system/os-win32.h
index b82a5d3ad9..cd61d69e10 100644
--- a/include/system/os-win32.h
+++ b/include/system/os-win32.h
@@ -123,8 +123,9 @@ static inline bool is_daemonized(void)
      return false;
  }

-static inline int os_mlock(void)

+static inline int os_mlock(bool on_fault)
  {
+    (void)on_fault;
      return -ENOSYS;
  }

diff --git a/meson.build b/meson.build

index 18cf9e2913..59953cbe6b 100644
--- a/meson.build
+++ b/meson.build
@@ -2885,6 +2885,12 @@ config_host_data.set('HAVE_MLOCKALL', 
cc.links(gnu_source_prefix + '''
      return mlockall(MCL_FUTURE);
    }'''))

+config_host_data.set('HAVE_MLOCK_ONFAULT', cc.links(gnu_source_prefix + '''

+  #include <sys/mman.h>
+  int main(void) {
+      return mlockall(MCL_FUTURE | MCL_ONFAULT);
+  }'''))
+
  have_l2tpv3 = false
  if get_option('l2tpv3').allowed() and have_system
    have_l2tpv3 = cc.has_type('struct mmsghdr',
diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index 6a6da6ba7f..fc4d8a10df 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -652,7 +652,7 @@ int postcopy_ram_incoming_cleanup(MigrationIncomingState 
*mis)
      }

if (enable_mlock) {

-        if (os_mlock() < 0) {
+        if (os_mlock(false) < 0) {
              error_report("mlock: %s", strerror(errno));
              /*
               * It doesn't feel right to fail at this point, we have a valid
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?

Sure, go ahead. Thanks!

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;
  #endif
+    }

ret = mlockall(flags);

      if (ret < 0) {

+
+    ret = mlockall(flags);
      if (ret < 0) {
          error_report("mlockall: %s", strerror(errno));
      }

return ret;

  #else
+    (void)on_fault;
      return -ENOSYS;
  #endif
  }
diff --git a/system/vl.c b/system/vl.c
index 9c6942c6cf..e94fc7ea35 100644
--- a/system/vl.c
+++ b/system/vl.c
@@ -797,7 +797,7 @@ static QemuOptsList qemu_run_with_opts = {
  static void realtime_init(void)
  {
      if (enable_mlock) {
-        if (os_mlock() < 0) {
+        if (os_mlock(false) < 0) {
              error_report("locking memory failed");
              exit(1);
          }
--
2.34.1