[Libguestfs] [PATCH 1/2] utils, builder: Add wrappers for posix_fadvise.

Richard W.M. Jones rjones at redhat.com
Thu Apr 14 13:54:18 UTC 2016


Add wrappers around posix_fadvise and use them in places we were
calling posix_fadvise directly before.

Also in virt-builder we were doing this (and ignoring the result):

  posix_fadvise (fd, 0, 0, POSIX_FADV_RANDOM|POSIX_FADV_DONTNEED);

However the POSIX_FADV_* flags are _not_ bitmasks!  In fact
POSIX_FADV_RANDOM|POSIX_FADV_DONTNEED == POSIX_FADV_NOREUSE so we were
giving a completely different hint from what we thought we were
giving.
---
 builder/pxzcat-c.c              |  5 +--
 src/guestfs-internal-frontend.h |  5 +++
 src/proto.c                     | 17 +-------
 src/utils.c                     | 87 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 95 insertions(+), 19 deletions(-)

diff --git a/builder/pxzcat-c.c b/builder/pxzcat-c.c
index 1f5ceeb..44722bc 100644
--- a/builder/pxzcat-c.c
+++ b/builder/pxzcat-c.c
@@ -214,10 +214,7 @@ pxzcat (value filenamev, value outputfilev, unsigned nr_threads)
     unix_error (err, (char *) "ftruncate", outputfilev);
   }
 
-#if defined HAVE_POSIX_FADVISE
-  /* Tell the kernel we won't read the output file. */
-  ignore_value (posix_fadvise (fd, 0, 0, POSIX_FADV_RANDOM|POSIX_FADV_DONTNEED));
-#endif
+  guestfs_int_fadvise_noreuse (fd);
 
   /* Iterate over blocks. */
   iter_blocks (idx, nr_threads, filenamev, fd, outputfilev, ofd);
diff --git a/src/guestfs-internal-frontend.h b/src/guestfs-internal-frontend.h
index 7f10906..1305105 100644
--- a/src/guestfs-internal-frontend.h
+++ b/src/guestfs-internal-frontend.h
@@ -87,6 +87,11 @@ extern int guestfs_int_is_true (const char *str);
 extern const char *guestfs_int_ovmf_i386_firmware[];
 extern const char *guestfs_int_ovmf_x86_64_firmware[];
 extern const char *guestfs_int_aavmf_firmware[];
+extern void guestfs_int_fadvise_sequential (int fd);
+extern void guestfs_int_fadvise_random (int fd);
+extern void guestfs_int_fadvise_noreuse (int fd);
+extern void guestfs_int_fadvise_dontneed (int fd);
+extern void guestfs_int_fadvise_willneed (int fd);
 
 /* These functions are used internally by the CLEANUP_* macros.
  * Don't call them directly.
diff --git a/src/proto.c b/src/proto.c
index 722ac9f..cbf8680 100644
--- a/src/proto.c
+++ b/src/proto.c
@@ -333,19 +333,6 @@ guestfs_int_send (guestfs_h *g, int proc_nr,
   return serial;
 }
 
-static void
-fadvise_sequential (int fd)
-{
-#if defined(HAVE_POSIX_FADVISE) && defined(POSIX_FADV_SEQUENTIAL)
-  /* Since the fd might be a non-file, eg. /dev/stdout, just ignore
-   * this when it fails.  It's not clear from the man page, but the
-   * 'advice' parameter is NOT a bitmask.  You can only pass one
-   * parameter with each call.
-   */
-  ignore_value (posix_fadvise (fd, 0, 0, POSIX_FADV_SEQUENTIAL));
-#endif
-}
-
 static int send_file_chunk (guestfs_h *g, int cancel, const char *buf, size_t len);
 static int send_file_data (guestfs_h *g, const char *buf, size_t len);
 static int send_file_cancellation (guestfs_h *g);
@@ -372,7 +359,7 @@ guestfs_int_send_file (guestfs_h *g, const char *filename)
     return -1;
   }
 
-  fadvise_sequential (fd);
+  guestfs_int_fadvise_sequential (fd);
 
   /* Send file in chunked encoding. */
   while (!g->user_cancel) {
@@ -829,7 +816,7 @@ guestfs_int_recv_file (guestfs_h *g, const char *filename)
     goto cancel;
   }
 
-  fadvise_sequential (fd);
+  guestfs_int_fadvise_sequential (fd);
 
   /* Receive the file in chunked encoding. */
   while ((r = receive_file_data (g, &buf)) > 0) {
diff --git a/src/utils.c b/src/utils.c
index a0e8f98..bbb2d12 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -35,6 +35,8 @@
 #include <sys/wait.h>
 #include <libintl.h>
 
+#include "ignore-value.h"
+
 /* NB: MUST NOT include "guestfs-internal.h" or gnulib headers. */
 #include "guestfs.h"
 #include "guestfs-internal-frontend.h"
@@ -385,3 +387,88 @@ guestfs_int_aavmf_firmware[] = {
 
   NULL
 };
+
+/**
+ * Hint that we will read or write the file descriptor sequentially.
+ *
+ * It's OK to call this on a non-file since we ignore failure as it is
+ * only a hint.
+ */
+void
+guestfs_int_fadvise_sequential (int fd)
+{
+#if defined(HAVE_POSIX_FADVISE) && defined(POSIX_FADV_SEQUENTIAL)
+  /* It's not clear from the man page, but the 'advice' parameter is
+   * NOT a bitmask.  You can only pass one parameter with each call.
+   */
+  ignore_value (posix_fadvise (fd, 0, 0, POSIX_FADV_SEQUENTIAL));
+#endif
+}
+
+/**
+ * Hint that we will read or write the file descriptor randomly.
+ *
+ * It's OK to call this on a non-file since we ignore failure as it is
+ * only a hint.
+ */
+void
+guestfs_int_fadvise_random (int fd)
+{
+#if defined(HAVE_POSIX_FADVISE) && defined(POSIX_FADV_RANDOM)
+  /* It's not clear from the man page, but the 'advice' parameter is
+   * NOT a bitmask.  You can only pass one parameter with each call.
+   */
+  ignore_value (posix_fadvise (fd, 0, 0, POSIX_FADV_RANDOM));
+#endif
+}
+
+/**
+ * Hint that we will access the data only once.
+ *
+ * It's OK to call this on a non-file since we ignore failure as it is
+ * only a hint.
+ */
+void
+guestfs_int_fadvise_noreuse (int fd)
+{
+#if defined(HAVE_POSIX_FADVISE) && defined(POSIX_FADV_NOREUSE)
+  /* It's not clear from the man page, but the 'advice' parameter is
+   * NOT a bitmask.  You can only pass one parameter with each call.
+   */
+  ignore_value (posix_fadvise (fd, 0, 0, POSIX_FADV_NOREUSE));
+#endif
+}
+
+/**
+ * Hint that we will not access the data in the near future.
+ *
+ * It's OK to call this on a non-file since we ignore failure as it is
+ * only a hint.
+ */
+void
+guestfs_int_fadvise_dontneed (int fd)
+{
+#if defined(HAVE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)
+  /* It's not clear from the man page, but the 'advice' parameter is
+   * NOT a bitmask.  You can only pass one parameter with each call.
+   */
+  ignore_value (posix_fadvise (fd, 0, 0, POSIX_FADV_DONTNEED));
+#endif
+}
+
+/**
+ * Hint that we will access the data in the near future.
+ *
+ * It's OK to call this on a non-file since we ignore failure as it is
+ * only a hint.
+ */
+void
+guestfs_int_fadvise_willneed (int fd)
+{
+#if defined(HAVE_POSIX_FADVISE) && defined(POSIX_FADV_WILLNEED)
+  /* It's not clear from the man page, but the 'advice' parameter is
+   * NOT a bitmask.  You can only pass one parameter with each call.
+   */
+  ignore_value (posix_fadvise (fd, 0, 0, POSIX_FADV_WILLNEED));
+#endif
+}
-- 
2.7.4




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