fuse: atomic_o_trunc should truncate pagecache

Fuse has an "atomic_o_trunc" mode, where userspace filesystem uses the
O_TRUNC flag in the OPEN request to truncate the file atomically with the
open.

In this mode there's no need to send a SETATTR request to userspace after
the open, so fuse_do_setattr() checks this mode and returns.  But this
misses the important step of truncating the pagecache.

Add the missing parts of truncation to the ATTR_OPEN branch.

Reported-by: Chad Austin <chadaustin@fb.com>
Fixes: 6ff958edbf39 ("fuse: add atomic open+truncate support")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org>

1 files changed, 12 insertions, 1 deletions

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 24967382a7b1..7a980b4462d9 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1629,8 +1629,19 @@ int fuse_do_setattr(struct dentry *dentry, struct iattr *attr,
                 return err;
 
         if (attr->ia_valid & ATTR_OPEN) {
-                if (fc->atomic_o_trunc)
+                /* This is coming from open(..., ... | O_TRUNC); */
+                WARN_ON(!(attr->ia_valid & ATTR_SIZE));
+                WARN_ON(attr->ia_size != 0);
+                if (fc->atomic_o_trunc) {
+                        /*
+                         * No need to send request to userspace, since actual
+                         * truncation has already been done by OPEN.  But still
+                         * need to truncate page cache.
+                         */
+                        i_size_write(inode, 0);
+                        truncate_pagecache(inode, 0);
                         return 0;
+                }
                 file = NULL;
         }