fuse: Trust kernel i_size only

Make fuse think that when writeback is on the inode's i_size is always
up-to-date and not update it with the value received from the userspace.
This is done because the page cache code may update i_size without letting
the FS know.

This assumption implies fixing the previously introduced short-read helper --
when a short read occurs the 'hole' is filled with zeroes.

fuse_file_fallocate() is also fixed because now we should keep i_size up to
date, so it must be updated if FUSE_FALLOCATE request succeeded.

Signed-off-by: Maxim V. Patlasov <MPatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

1 files changed, 9 insertions, 2 deletions

diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index d468643a68b2..c668c8436894 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -197,6 +197,7 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
 {
         struct fuse_conn *fc = get_fuse_conn(inode);
         struct fuse_inode *fi = get_fuse_inode(inode);
+        bool is_wb = fc->writeback_cache;
         loff_t oldsize;
         struct timespec old_mtime;
 
@@ -211,10 +212,16 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
         fuse_change_attributes_common(inode, attr, attr_valid);
 
         oldsize = inode->i_size;
-        i_size_write(inode, attr->size);
+        /*
+         * In case of writeback_cache enabled, the cached writes beyond EOF
+         * extend local i_size without keeping userspace server in sync. So,
+         * attr->size coming from server can be stale. We cannot trust it.
+         */
+        if (!is_wb || !S_ISREG(inode->i_mode))
+                i_size_write(inode, attr->size);
         spin_unlock(&fc->lock);
 
-        if (S_ISREG(inode->i_mode)) {
+        if (!is_wb && S_ISREG(inode->i_mode)) {
                 bool inval = false;
 
                 if (oldsize != attr->size) {