From 41a86d41a9b2b937faeda6cafc225d50f266a0ad Mon Sep 17 00:00:00 2001
From: Jan Wassenberg <janwas@google.com>
Date: Fri, 15 Aug 2025 06:30:07 -0700
Subject: [PATCH] Fix preadv error: only enable if we have a handle

PiperOrigin-RevId: 795455020
---
 io/io.cc | 30 +++++++++++++++++-------------
 io/io.h  |  2 +-
 2 files changed, 18 insertions(+), 14 deletions(-)

diff --git a/io/io.cc b/io/io.cc
index df39f7b..d19ec10 100644
--- a/io/io.cc
+++ b/io/io.cc
@@ -226,21 +226,26 @@ void InternalInit() {
 }
 
 uint64_t IOBatch::Read(const File& file) const {
-#if GEMMA_IO_PREADV
   HWY_ASSERT(!spans_.empty());
 
-  ssize_t bytes_read;
-  for (;;) {
-    bytes_read =
-        preadv(file.Handle(), reinterpret_cast<const iovec*>(spans_.data()),
-               static_cast<int>(spans_.size()), offset_);
-    if (bytes_read >= 0) break;
-    if (errno == EINTR) continue;  // signal: retry
-    HWY_WARN("preadv failed, errno %d.", errno);
-    return 0;
+#if GEMMA_IO_PREADV
+  if (file.Handle() != -1) {
+    ssize_t bytes_read;
+    for (;;) {
+      bytes_read =
+          preadv(file.Handle(), reinterpret_cast<const iovec*>(spans_.data()),
+                 static_cast<int>(spans_.size()), offset_);
+      if (bytes_read >= 0) break;
+      if (errno == EINTR) continue;  // signal: retry
+      HWY_WARN("preadv(%d) for %4zu spans from offset %12zu failed, errno %d.",
+               file.Handle(), spans_.size(), offset_, errno);
+      return 0;
+    }
+    return static_cast<uint64_t>(bytes_read);
   }
-  return static_cast<uint64_t>(bytes_read);
-#else
+#endif  // GEMMA_IO_PREADV
+
+  // preadv disabled or no handle: use normal reads (higher kernel overhead).
   uint64_t total = 0;
   uint64_t offset = offset_;
   for (const IOSpan& span : spans_) {
@@ -249,7 +254,6 @@ uint64_t IOBatch::Read(const File& file) const {
     offset += span.bytes;
   }
   return total;
-#endif
 }
 
 }  // namespace gcpp
diff --git a/io/io.h b/io/io.h
index d9481bc..f90a636 100644
--- a/io/io.h
+++ b/io/io.h
@@ -68,7 +68,7 @@ class File {
   // modify internal state. This is only expected to be called once per file.
   virtual MapPtr Map() = 0;
 
-  // For use by `IOBatch::Read`.
+  // Returns handle for use by `IOBatch::Read`, or -1 if not supported.
   virtual int Handle() const { return -1; }
 };