fuse: enable per inode DAX

DAX may be limited in some specific situation. When the number of usable
DAX windows is under watermark, the recalim routine will be triggered to
reclaim some DAX windows. It may have a negative impact on the
performance, since some processes may need to wait for DAX windows to be
recalimed and reused then. To mitigate the performance degradation, the
overall DAX window need to be expanded larger.

However, simply expanding the DAX window may not be a good deal in some
scenario. To maintain one DAX window chunk (i.e., 2MB in size), 32KB
(512 * 64 bytes) memory footprint will be consumed for page descriptors
inside guest, which is greater than the memory footprint if it uses
guest page cache when DAX disabled. 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.

Per inode DAX feature is introduced to address this issue, by offering a
finer grained control for dax to users, trying to achieve a balance
between performance and memory overhead.

The FUSE_ATTR_DAX flag in FUSE_LOOKUP reply is used to indicate whether
DAX should be enabled or not for corresponding file. Currently the state
whether DAX is enabled or not for the file is initialized only when
inode is instantiated.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 8 insertions, 4 deletions

diff --git a/fs/fuse/dax.c b/fs/fuse/dax.c
index d2bc5e7f5132..ff17bc7a0b6e 100644
--- a/fs/fuse/dax.c
+++ b/fs/fuse/dax.c
@@ -1330,7 +1330,7 @@ static const struct address_space_operations fuse_dax_file_aops  = {
 	.invalidatepage	= noop_invalidatepage,
 };
 
-static bool fuse_should_enable_dax(struct inode *inode)
+static bool fuse_should_enable_dax(struct inode *inode, unsigned int flags)
 {
 	struct fuse_conn *fc = get_fuse_conn(inode);
 	enum fuse_dax_mode dax_mode = fc->dax_mode;
@@ -1345,12 +1345,16 @@ static bool fuse_should_enable_dax(struct inode *inode)
 	if (!fc->dax)
 		return false;
 
-	return true;
+	if (dax_mode == FUSE_DAX_ALWAYS)
+		return true;
+
+	/* dax_mode is FUSE_DAX_INODE* */
+	return flags & FUSE_ATTR_DAX;
 }
 
-void fuse_dax_inode_init(struct inode *inode)
+void fuse_dax_inode_init(struct inode *inode, unsigned int flags)
 {
-	if (!fuse_should_enable_dax(inode))
+	if (!fuse_should_enable_dax(inode, flags))
 		return;
 
 	inode->i_flags |= S_DAX;