fuse: add support for kernel writeback cache

- Added kernel-writeback-cache command line and xlator
  option for requesting utilisation of the writeback
  cache of the kernel in FUSE_INIT (see [1]).
- Added attr-times-granularity command line and xlator
  option via which granularity of the {a,m,c}time in
  stat (attr) data that we support can be indicated to
  kernel. This is a means to avoid divergence of the
  attr times between kernel and userspace that could
  occur with writeback-cache, while still maintaining
  maximum time precision the FUSE server is capable of
  (see [2]).
- Handling FATTR_CTIME flag in FUSE_SETATTR that
  indicates presence of ctime in setattr payload.
  Currently we cannot associate arbitrary ctimes to
  files on backend, so we just touch them to update
  their ctimes to current time. Having ctimes in setattr
  payload is also a side effect of writeback cache
  (see [3] and [4]).

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4d99ff8,
     "fuse: Turn writeback cache on"
[2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e27c9d3,
     "fuse: fuse: add time_gran to INIT_OUT"
[3]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1e18bda,
     "fuse: add .write_inode"
[4]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ab9e13f,
     "fuse: allow ctime flushing to userspace"

Updates: #435
Change-Id: Id174c8e0c815c4456c35f8c53e41a6a507d91855
Signed-off-by: Csaba Henk <csaba@redhat.com>

2 files changed, 5 insertions, 0 deletions

diff --git a/libglusterfs/src/glusterfs.h b/libglusterfs/src/glusterfs.h
index 904d4b60d75..90f2762b990 100644
--- a/libglusterfs/src/glusterfs.h
+++ b/libglusterfs/src/glusterfs.h
@@ -532,6 +532,10 @@ struct _cmd_args {
         char              *event_history;
         int                thin_client;
         uint32_t           reader_thread_count;
+
+        /* FUSE writeback cache support */
+        int                kernel_writeback_cache;
+        uint32_t           attr_times_granularity;
 };
 typedef struct _cmd_args cmd_args_t;
 
diff --git a/libglusterfs/src/xlator.h b/libglusterfs/src/xlator.h
index 4f18d1cd2a9..d476cf26442 100644
--- a/libglusterfs/src/xlator.h
+++ b/libglusterfs/src/xlator.h
@@ -33,6 +33,7 @@
 #define GF_SET_ATTR_SIZE  0x8
 #define GF_SET_ATTR_ATIME 0x10
 #define GF_SET_ATTR_MTIME 0x20
+#define GF_SET_ATTR_CTIME 0x40
 
 #define gf_attr_mode_set(mode)  ((mode) & GF_SET_ATTR_MODE)
 #define gf_attr_uid_set(mode)   ((mode) & GF_SET_ATTR_UID)