From: Carlos Maiolino <cmaiolino-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Date: Thu, 20 Apr 2017 14:53:01 +0200
Subject: [PATCH] make buffer size match kernel max transfer size

Currently libfuse has a hardcoded buffer limit to 128kib, while fuse
kernel module has a limit up to 32 pages.

This patch changes buffer limit to match the current page size, instead
of assuming 4096 bytes pages, enabling architectures with bigger pages
to use larger buffers, improving performance.

Also, add a new macro (HEADER_SIZE) to specify the space needed to
accommodate the header, making it easier to understand why those extra
4096 bytes are needed

Signed-off-by: Carlos Maiolino <cmaiolino-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>

diff --git a/lib/fuse_kern_chan.c b/lib/fuse_kern_chan.c
index 4a9beb8..640b91a 100644
--- a/lib/fuse_kern_chan.c
+++ b/lib/fuse_kern_chan.c
@@ -83,7 +83,10 @@ static void fuse_kern_chan_destroy(struct fuse_chan *ch)
 		close(fd);
 }
 
-#define MIN_BUFSIZE 0x21000
+#define KERNEL_BUF_PAGES 32
+
+/* room needed in buffer to accommodate header */
+#define HEADER_SIZE 0x1000
 
 struct fuse_chan *fuse_kern_chan_new(int fd)
 {
@@ -92,7 +95,6 @@ struct fuse_chan *fuse_kern_chan_new(int fd)
 		.send = fuse_kern_chan_send,
 		.destroy = fuse_kern_chan_destroy,
 	};
-	size_t bufsize = getpagesize() + 0x1000;
-	bufsize = bufsize < MIN_BUFSIZE ? MIN_BUFSIZE : bufsize;
+	size_t bufsize = KERNEL_BUF_PAGES * getpagesize() + HEADER_SIZE;
 	return fuse_chan_new(&op, fd, bufsize, NULL);
 }