gh-149584: _remote_debugging: Map remote pages with mach_vm_remap on macOS instead of copying them

Misc/NEWS.d/next/Library/2026-10-31-16-46-56.gh-issue-149584.NFB5Ts.rst

@@ -0,0 +1,3 @@
+Speed up :mod:`profiling.sampling` on macOS by mapping the target process's
+memory pages into the profiler with ``mach_vm_remap`` instead of copying
+them on every sample.

Modules/_remote_debugging/frame_cache.c

@@ -167,10 +167,18 @@ read_last_profiled_anchor(RemoteUnwinderObject *unwinder,
     // remote-memory read.
     if (seq_offset == frame_offset + sizeof(uintptr_t)) {
         uintptr_t live_anchor[2];
-        if (_Py_RemoteDebug_ReadRemoteMemory(&unwinder->handle,
-                                             thread_state_addr + frame_offset,
-                                             sizeof(live_anchor),
-                                             live_anchor) < 0) {
+        int rc = _Py_RemoteDebug_AliasedReadRemoteMemory(&unwinder->handle,
+                                                        thread_state_addr + frame_offset,
+                                                        sizeof(live_anchor),
+                                                        live_anchor);
+        if (rc < 0) {
+            return -1;
+        }
+        if (rc > 0
+            && _Py_RemoteDebug_ReadRemoteMemory(&unwinder->handle,
+                                                thread_state_addr + frame_offset,
+                                                sizeof(live_anchor),
+                                                live_anchor) < 0) {
             return -1;
         }
         anchor->frame = live_anchor[0];

Modules/_remote_debugging/frames.c

@@ -36,7 +36,7 @@ process_single_stack_chunk(
         return -1;
     }
 
-    if (_Py_RemoteDebug_PagedReadRemoteMemory(&unwinder->handle, chunk_addr, current_size, this_chunk) < 0) {
+    if (_Py_RemoteDebug_ReadRemoteMemory(&unwinder->handle, chunk_addr, current_size, this_chunk) < 0) {
         PyMem_RawFree(this_chunk);
         set_exception_cause(unwinder, PyExc_RuntimeError, "Failed to read stack chunk");
         return -1;
@@ -65,7 +65,7 @@ process_single_stack_chunk(
         }
         this_chunk = tmp;
 
-        if (_Py_RemoteDebug_PagedReadRemoteMemory(&unwinder->handle, chunk_addr, actual_size, this_chunk) < 0) {
+        if (_Py_RemoteDebug_ReadRemoteMemory(&unwinder->handle, chunk_addr, actual_size, this_chunk) < 0) {
             PyMem_RawFree(this_chunk);
             set_exception_cause(unwinder, PyExc_RuntimeError, "Failed to reread stack chunk with correct size");
             return -1;

Modules/_remote_debugging/module.c

@@ -608,6 +608,27 @@ read_interp_state_and_maybe_thread_frame(
 {
     prefetch->tstate = NULL;
     prefetch->frame = NULL;
+    int aliased = _Py_RemoteDebug_AliasedReadRemoteMemory(
+        &unwinder->handle, interpreter_addr, INTERP_STATE_BUFFER_SIZE,
+        interp_state_buffer);
+    if (aliased < 0) {
+        return -1;
+    }
+    if (aliased == 0) {
+        if (prefetch->tstate_addr != 0) {
+            size_t tstate_size = (size_t)unwinder->debug_offsets.thread_state.size;
+            int rc = _Py_RemoteDebug_AliasedReadRemoteMemory(
+                &unwinder->handle, prefetch->tstate_addr, tstate_size,
+                tstate_buffer);
+            if (rc == 0) {
+                prefetch->tstate = tstate_buffer;
+            }
+            else if (rc < 0) {
+                PyErr_Clear();
+            }
+        }
+        return 0;
+    }
     if (prefetch->tstate_addr != 0) {
         size_t tstate_size = (size_t)unwinder->debug_offsets.thread_state.size;
         _Py_RemoteReadSegment segments[3] = {

Modules/_remote_debugging/threads.c

@@ -304,6 +304,11 @@ read_thread_state_and_maybe_frame(
     int *frame_read)
 {
     *frame_read = 0;
+    int aliased = _Py_RemoteDebug_AliasedReadRemoteMemory(
+        &unwinder->handle, tstate_addr, tstate_size, tstate_buffer);
+    if (aliased <= 0) {
+        return aliased;
+    }
     if (predicted_frame_addr != 0) {
         _Py_RemoteReadSegment segments[2] = {
             {tstate_addr, tstate_buffer, tstate_size},
@@ -380,11 +385,15 @@ unwind_stack_for_thread(
         }
     }
     else {
-        int rc = _Py_RemoteDebug_ReadRemoteMemory(
-            &unwinder->handle,
+        int unused_frame_read = 0;
+        int rc = read_thread_state_and_maybe_frame(
+            unwinder,
             *current_tstate,
             (size_t)unwinder->debug_offsets.thread_state.size,
-            local_ts);
+            local_ts,
+            0,
+            NULL,
+            &unused_frame_read);
         if (rc < 0) {
             set_exception_cause(unwinder, PyExc_RuntimeError, "Failed to read thread state");
             goto error;