From df0e91d488276086bc07da2e389986cae0048c37 Mon Sep 17 00:00:00 2001
From: Miklos Szeredi <mszeredi@redhat.com>
Date: Thu, 8 Feb 2018 15:17:38 +0100
Subject: [PATCH] fuse: atomic_o_trunc should truncate pagecache
Git-commit: df0e91d488276086bc07da2e389986cae0048c37
Patch-mainline: v4.18-rc1
References: bsc#1051510

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>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 fs/fuse/dir.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

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;
 	}
 
-- 
2.18.0