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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org, "Miklos Szeredi" <mszeredi@redhat.com>,
        "Chad Austin" <chadaustin@fb.com>
Date:   Sun, 11 Nov 2018 19:49:05 +0000
Message-ID: <lsq.1541965745.693257686@decadent.org.uk>
Subject: [PATCH 3.16 014/366] fuse: atomic_o_trunc should truncate pagecache
In-Reply-To: <lsq.1541965744.387173642@decadent.org.uk>
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3.16.61-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Miklos Szeredi <mszeredi@redhat.com>

commit df0e91d488276086bc07da2e389986cae0048c37 upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 fs/fuse/dir.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1728,8 +1728,19 @@ int fuse_do_setattr(struct dentry *dentr
 		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;
 	}