Changeset 85492 in vbox for trunk/src/VBox/Runtime/r3/posix

Timestamp:

Jul 28, 2020 4:36:42 PM (5 years ago)

Author:

vboxsync

svn:sync-xref-src-repo-rev:

139548

Message:

IPRT/fileio-posix.cpp: Fix binary search in RTFileQueryMaxSizeEx, and make it cover a bigger range (ext4 handles almost 16TB per file), with the help of a quicker "find the approximate order of magnitude" approach all the way up to INT64_MAX, which is the theoretical maximum for the seekable range in many OSes anyway.

File:

• trunk/src/VBox/Runtime/r3/posix/fileio-posix.cpp (modified) (1 diff)

trunk/src/VBox/Runtime/r3/posix/fileio-posix.cpp

@@ -756 +756 @@
         return rc;
 
-    /*
-     * Perform a binary search for the max file size.
-     */
     uint64_t offLow  =       0;
-    uint64_t offHigh = 8 * _1T; /* we don't need bigger files */
+    uint64_t offHigh = INT64_MAX; /* we don't need bigger files */
     /** @todo Unfortunately this does not work for certain file system types,
      * for instance cifs mounts. Even worse, statvfs.f_fsid returns 0 for such
      * file systems. */
-    //uint64_t offHigh = INT64_MAX;
-    for (;;)
-    {
-        uint64_t cbInterval = (offHigh - offLow) >> 1;
-        if (cbInterval == 0)
-        {
-            if (pcbMax)
-                *pcbMax = offLow;
-            return RTFileSeek(hFile, offOld, RTFILE_SEEK_BEGIN, NULL);
-        }
-
-        rc = RTFileSeek(hFile, offLow + cbInterval, RTFILE_SEEK_BEGIN, NULL);
+
+    /*
+     * Quickly guess the order of magnitude for offHigh and offLow.
+     */
+    {
+        uint64_t offHighPrev = offHigh;
+        while (offHigh >= INT32_MAX)
+        {
+            rc = RTFileSeek(hFile, offHigh, RTFILE_SEEK_BEGIN, NULL);
+            if (RT_SUCCESS(rc))
+            {
+                offLow = offHigh;
+                offHigh = offHighPrev;
+                break;
+            }
+            else
+            {
+                offHighPrev = offHigh;
+                offHigh >>= 8;
+            }
+        }
+    }
+
+    /*
+     * Sanity: if the seek to the initial offHigh (INT64_MAX) works, then
+     * this algorithm cannot possibly work. Declare defeat.
+     */
+    if (offLow == offHigh)
+        return VERR_NOT_SUPPORTED;
+
+    /*
+     * Perform a binary search for the max file size.
+     */
+    while (offLow <= offHigh)
+    {
+        uint64_t offMid = offLow + (offHigh - offLow) / 2;
+        rc = RTFileSeek(hFile, offMid, RTFILE_SEEK_BEGIN, NULL);
         if (RT_FAILURE(rc))
-            offHigh = offLow + cbInterval;
+            offHigh = offMid - 1;
         else
-            offLow  = offLow + cbInterval;
-    }
+            offLow  = offMid + 1;
+    }
+
+    if (pcbMax)
+        *pcbMax = RT_MIN(offLow, offHigh);
+    return RTFileSeek(hFile, offOld, RTFILE_SEEK_BEGIN, NULL);
 }