[ExecuTorch][MmapDataLoader] Issue F_RDADVISE on Apple platforms in UseMadvise path

extension/data_loader/mman.h

@@ -17,6 +17,7 @@
 
 #ifndef _WIN32
 
+#include <fcntl.h>
 #include <sys/mman.h>
 #include <unistd.h>
 
@@ -53,6 +54,24 @@ ET_INLINE void madvise_pages_willneed_sequential(void* addr, size_t len) {
   ::madvise(addr, len, MADV_SEQUENTIAL);
 }
 
+/**
+ * On Apple platforms, schedule kernel read-ahead on the file descriptor itself
+ * via fcntl(F_RDADVISE). This is more aggressive than madvise for cold starts:
+ * it brings pages into the unified buffer cache so first-touch faults are
+ * serviced from RAM instead of storage. No-op on non-Apple POSIX platforms.
+ */
+ET_INLINE void fcntl_rdadvise_apple(int fd, size_t file_size) {
+#if defined(__APPLE__)
+  struct radvisory advice;
+  advice.ra_offset = 0;
+  advice.ra_count = static_cast<int>(file_size);
+  ::fcntl(fd, F_RDADVISE, &advice);
+#else
+  (void)fd;
+  (void)file_size;
+#endif
+}
+
 #else
 
 #define NOMINMAX
@@ -99,4 +118,12 @@ ET_INLINE void madvise_pages_willneed_sequential(void* addr, size_t len) {
   (void)len;
 }
 
+/**
+ * No-op on Windows: F_RDADVISE is an Apple-specific fcntl command.
+ */
+ET_INLINE void fcntl_rdadvise_apple(int fd, size_t file_size) {
+  (void)fd;
+  (void)file_size;
+}
+
 #endif

extension/data_loader/mmap_data_loader.cpp

@@ -251,6 +251,7 @@ Result<FreeableBuffer> MmapDataLoader::load(
 
   if (mlock_config_ == MlockConfig::UseMadvise) {
     madvise_pages_willneed_sequential(pages, map_size);
+    fcntl_rdadvise_apple(fd_, file_size_);
   }
 
   // The requested data is at an offset into the mapped pages.