[Virtio-fs] [PATCH v7 6/6] virtiofsd: implement file size based dax policy

[Jeffle Xu]

When DAX window is fully utilized and needs to be expanded to avoid the
performance fluctuation triggered by DAX window recaliming, it may not
be wise to allocate DAX window for files with size smaller than some
specific point, considering from the perspective of reducing memory
overhead.

To maintain one DAX window chunk (e.g., 2MB in size), 32KB
(512 * 64 bytes) memory footprint will be consumed for page descriptors
inside guest. Thus it'd better disable DAX for those files smaller than
32KB, to reduce the demand for DAX window and thus avoid the unworthy
memory overhead.

Thus only flag the file with FUSE_ATTR_DAX when the file size is greater
than 32 KB. The guest will enable DAX only for those files flagged with
FUSE_ATTR_DAX, when virtiofs is mounted with '-o dax=inode'.

To be noted that both FUSE_LOOKUP and FUSE_READDIRPLUS are affected, and
will convey FUSE_ATTR_DAX flag to the guest.

This policy will be used when '-o dax=filesize' option is specified for
virtiofsd.

Signed-off-by: Jeffle Xu <jefflexu at linux.alibaba.com>
---
 tools/virtiofsd/passthrough_ll.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index dac5063594..cbf81d1593 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -54,6 +54,7 @@
 #include <sys/syscall.h>
 #include <sys/wait.h>
 #include <sys/xattr.h>
+#include <sys/user.h>
 #include <syslog.h>
 #include <linux/fs.h>
 
@@ -61,6 +62,20 @@
 #include "passthrough_helpers.h"
 #include "passthrough_seccomp.h"
 
+/*
+ * One page descriptor (64 bytes in size) needs to be maintained for every page
+ * in the DAX window chunk, i.e., there is certain guest memory overhead when
+ * DAX is enabled. Thus disable DAX for those with file size smaller than this
+ * certain memory overhead if virtiofs is mounted in per inode DAX mode. In
+ * this case, the guest page cache will consume less memory than that when DAX
+ * is enabled.
+ */
+#define FUSE_DAX_SHIFT      21
+#define PAGE_DESC_SHIFT     6
+#define FUSE_INODE_DAX_SHIFT \
+    (FUSE_DAX_SHIFT - PAGE_SHIFT + PAGE_DESC_SHIFT)
+#define FUSE_INODE_DAX_THRESH  (1 << FUSE_INODE_DAX_SHIFT)
+
 /* Keep track of inode posix locks for each owner. */
 struct lo_inode_plock {
     uint64_t lock_owner;
@@ -1052,6 +1067,10 @@ static bool lo_should_enable_dax(struct lo_data *lo, struct lo_inode *inode,
         return ret;
     }
 
+    if (lo->dax == INODE_DAX_FILESIZE) {
+        return e->attr.st_size > FUSE_INODE_DAX_THRESH;
+    }
+
     return false;
 }
 
-- 
2.27.0