file: support for XFS on older kernels

Linux 3.15+ and some RHEL 7 kernel versions support XFS AIO append, but
others do not.  Try to see if the current kernel support XFS AIO append
and enable it, and fall back to ftruncate if it does not.  This allows
us to run on older kernels without significant performance loss.

Message-Id: <[email protected]>

core/reactor.cc

@@ -76,6 +76,7 @@
 #endif
 
 #include <sys/mman.h>
+#include <sys/utsname.h>
 #include <linux/falloc.h>
 #include <linux/magic.h>
 #include "util/backtrace.hh"
@@ -1190,6 +1191,49 @@ append_challenged_posix_file_impl::close() noexcept {
     });
 }
 
+// Some kernels can append to xfs filesystems, some cannot; determine
+// from kernel version.
+static
+unsigned
+xfs_concurrency_from_kernel_version() {
+    auto num = [] (std::csub_match x) {
+        auto b = x.first;
+        auto e = x.second;
+        if (*b == '.') {
+            ++b;
+        }
+        return std::stoi(std::string(b, e));
+    };
+    struct utsname buf;
+    auto r = ::uname(&buf);
+    throw_system_error_on(r == -1);
+    // 2-4 dotted decimal numbers, optional "-anything"
+    auto generic_re = std::regex(R"XX((\d+)(\.\d+)(\.\d+)?(\.\d+)?(-.*)?)XX");
+    std::cmatch m1;
+    // try to see if this is a mainline kernel with xfs append fixed (3.15+)
+    if (std::regex_match(buf.release, m1, generic_re)) {
+        auto maj = num(m1[1]);
+        auto min = num(m1[2]);
+        if (maj > 3 || (maj == 3 && min >= 15)) {
+            // Can append, but not concurrently
+            return 1;
+        }
+    }
+    // 3.10.0-num1.num2?.num3?.el7.anything
+    auto rhel_re = std::regex(R"XX(3\.10\.0-(\d+)(\.\d+)?(\.\d+)?\.el7.*)XX");
+    std::cmatch m2;
+    // try to see if this is a RHEL kernel with the backported fix (3.10.0-325.el7+)
+    if (std::regex_match(buf.release, m2, rhel_re)) {
+        auto rmaj = num(m2[1]);
+        if (rmaj >= 325) {
+            // Can append, but not concurrently
+            return 1;
+        }
+    }
+    // Cannot append at all; need ftrucnate().
+    return 0;
+}
+
 inline
 shared_ptr<file_impl>
 make_file_impl(int fd, file_open_options options) {
@@ -1222,7 +1266,8 @@ make_file_impl(int fd, file_open_options options) {
             switch (sfs.f_type) {
             case 0x58465342: /* XFS */
                 as.append_challenged = true;
-                as.append_concurrency = 1;
+                static auto xc = xfs_concurrency_from_kernel_version();
+                as.append_concurrency = xc;
                 break;
             case 0x6969: /* NFS */
                 as.append_challenged = false;