[PATCH] fuse: fix async read for legacy filesystems

While asynchronous reads mean a performance improvement in most cases, if
the filesystem assumed that reads are synchronous, then async reads may
degrade performance (filesystem may receive reads out of order, which can
confuse it's own readahead logic).

With sshfs a 1.5 to 4 times slowdown can be measured.

There's also a need for userspace filesystems to know whether asynchronous
reads are supported by the kernel or not.

To achive these, negotiate in the INIT request whether async reads will be
used and the maximum readahead value.  Update interface version to 7.6

If userspace uses a version earlier than 7.6, then disable async reads, and
set maximum readahead value to the maximum read size, as done in previous
versions.

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 14 insertions, 2 deletions

diff --git a/include/linux/fuse.h b/include/linux/fuse.h
index 528959c52f1b..5425b60021e3 100644
--- a/include/linux/fuse.h
+++ b/include/linux/fuse.h
@@ -14,7 +14,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 5
+#define FUSE_KERNEL_MINOR_VERSION 6
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -58,6 +58,9 @@ struct fuse_kstatfs {
         __u32   spare[6];
 };
 
+/**
+ * Bitmasks for fuse_setattr_in.valid
+ */
 #define FATTR_MODE      (1 << 0)
 #define FATTR_UID       (1 << 1)
 #define FATTR_GID       (1 << 2)
@@ -75,6 +78,11 @@ struct fuse_kstatfs {
 #define FOPEN_DIRECT_IO         (1 << 0)
 #define FOPEN_KEEP_CACHE        (1 << 1)
 
+/**
+ * INIT request/reply flags
+ */
+#define FUSE_ASYNC_READ         (1 << 0)
+
 enum fuse_opcode {
         FUSE_LOOKUP        = 1,
         FUSE_FORGET        = 2,  /* no reply */
@@ -247,12 +255,16 @@ struct fuse_access_in {
 struct fuse_init_in {
         __u32   major;
         __u32   minor;
+        __u32   max_readahead;
+        __u32   flags;
 };
 
 struct fuse_init_out {
         __u32   major;
         __u32   minor;
-        __u32   unused[3];
+        __u32   max_readahead;
+        __u32   flags;
+        __u32   unused;
         __u32   max_write;
 };